refactor: extract psysonic-syncfs crate (M4/7)

Moves all on-disk cache + device-sync code out of the top crate:

  crates/psysonic-syncfs/                      new lib crate
    src/cache/{offline,hot,downloads,fs_utils} unchanged behaviour
    src/sync/{batch,device}                    (no tray.rs — that's UI)
    src/file_transfer.rs                       shared HTTP helpers
    src/lib.rs                                 + DownloadSemaphore type
                                               + sync_cancel_flags fn

The shell crate keeps `lib_commands/sync/tray.rs` (OS tray icon — UI
concern, will move to shell-tauri at M7) but drops the rest of
`lib_commands/cache/` and the syncfs sibling files.

Tauri command quirk surfaced and resolved: `#[tauri::command]` puts its
`__cmd__*` and `__tauri_command_name_*` helper macros at the *exact*
module of the function, and `pub use` doesn't carry them across module
boundaries. invoke_handler! in lib.rs now references each syncfs
command via its full deepest path
(`psysonic_syncfs::cache::offline::download_track_offline`, etc.) so
Tauri's macros resolve at the right scope. Same approach the audio
crate already uses (`audio::commands::audio_play`).

Cross-crate ref migrations applied via batch sed:
  crate::audio::*               → psysonic_audio::*
  crate::analysis_runtime::*    → psysonic_analysis::analysis_runtime::*
  crate::analysis_cache::*      → psysonic_analysis::analysis_cache::*
  crate::subsonic_wire_user_agent → psysonic_core::user_agent::*
  super::super::file_transfer:: → crate::file_transfer::

Behaviour preserving. Cargo check + clippy --workspace clean.

src-tauri/crates/psysonic-syncfs/Cargo.toml

@@ -0,0 +1,25 @@
+[package]
+name = "psysonic-syncfs"
+version.workspace = true
+edition.workspace = true
+rust-version.workspace = true
+publish = false
+
+[dependencies]
+psysonic-core = { path = "../psysonic-core" }
+psysonic-analysis = { path = "../psysonic-analysis" }
+psysonic-audio = { path = "../psysonic-audio" }
+
+tauri = { version = "2" }
+tauri-plugin-shell = "2"
+tauri-plugin-process = "2"
+serde = { version = "1", features = ["derive"] }
+serde_json = "1"
+tokio = { version = "1", features = ["rt", "time", "sync"] }
+reqwest = { version = "0.13", default-features = false, features = ["stream", "json", "rustls", "blocking"] }
+futures-util = "0.3"
+sysinfo = { version = "0.38", default-features = false, features = ["disk"] }
+url = "2"
+md5 = "0.8"
+lofty = "0.24"
+id3 = "1.16.4"

src-tauri/crates/psysonic-syncfs/src/cache/downloads.rs

@@ -0,0 +1,404 @@
+use tauri::{Emitter, Manager};
+
+use psysonic_core::user_agent::subsonic_wire_user_agent;
+
+pub fn resolve_hot_cache_root(
+    custom_dir: Option<String>,
+    app: &tauri::AppHandle,
+) -> Result<std::path::PathBuf, String> {
+    if let Some(ref cd) = custom_dir.filter(|s| !s.is_empty()) {
+        let base = std::path::PathBuf::from(cd);
+        if !base.exists() {
+            return Err("VOLUME_NOT_FOUND".to_string());
+        }
+        Ok(base.join("psysonic-hot-cache"))
+    } else {
+        Ok(app
+            .path()
+            .app_data_dir()
+            .map_err(|e| e.to_string())?
+            .join("psysonic-hot-cache"))
+    }
+}
+
+/// Returns true if the current Linux system is Arch-based
+/// (checks /etc/arch-release and /etc/os-release).
+#[tauri::command]
+pub fn check_arch_linux() -> bool {
+    #[cfg(target_os = "linux")]
+    {
+        if std::path::Path::new("/etc/arch-release").exists() {
+            return true;
+        }
+        if let Ok(content) = std::fs::read_to_string("/etc/os-release") {
+            for line in content.lines() {
+                let lower = line.to_lowercase();
+                if lower.starts_with("id=arch") { return true; }
+                if lower.starts_with("id_like=") && lower.contains("arch") { return true; }
+            }
+        }
+        false
+    }
+    #[cfg(not(target_os = "linux"))]
+    { false }
+}
+
+/// Progress payload emitted during an update binary download.
+#[derive(Clone, serde::Serialize)]
+#[serde(rename_all = "camelCase")]
+pub struct UpdateDownloadProgress {
+    bytes: u64,
+    total: Option<u64>,
+}
+
+/// Downloads an update installer/package to the user's Downloads folder.
+/// Emits `update:download:progress` events with `{ bytes, total }` every 250 ms.
+/// Returns the final absolute file path on success.
+#[tauri::command]
+pub async fn download_update(url: String, filename: String, app: tauri::AppHandle) -> Result<String, String> {
+    use futures_util::StreamExt;
+    use std::time::{Duration, Instant};
+    use tokio::io::AsyncWriteExt;
+
+    const EMIT_INTERVAL: Duration = Duration::from_millis(250);
+
+    let dest_dir = app.path().download_dir().map_err(|e| e.to_string())?;
+    let dest_path = dest_dir.join(&filename);
+    let part_path = dest_dir.join(format!("{}.part", filename));
+
+    let client = reqwest::Client::builder()
+        .user_agent(subsonic_wire_user_agent())
+        .connect_timeout(Duration::from_secs(30))
+        .timeout(Duration::from_secs(3600))
+        .build()
+        .map_err(|e| e.to_string())?;
+
+    let response = client.get(&url).send().await.map_err(|e| e.to_string())?;
+    if !response.status().is_success() {
+        return Err(format!("HTTP {}", response.status().as_u16()));
+    }
+
+    let total = response.content_length();
+
+    let result: Result<u64, String> = async {
+        let mut file = tokio::fs::File::create(&part_path)
+            .await
+            .map_err(|e| e.to_string())?;
+
+        let mut bytes_done: u64 = 0;
+        let mut stream = response.bytes_stream();
+        let mut last_emit = Instant::now();
+
+        while let Some(chunk) = stream.next().await {
+            let chunk = chunk.map_err(|e| e.to_string())?;
+            file.write_all(&chunk).await.map_err(|e| e.to_string())?;
+            bytes_done += chunk.len() as u64;
+
+            if last_emit.elapsed() >= EMIT_INTERVAL {
+                let _ = app.emit("update:download:progress", UpdateDownloadProgress {
+                    bytes: bytes_done,
+                    total,
+                });
+                last_emit = Instant::now();
+            }
+        }
+        file.flush().await.map_err(|e| e.to_string())?;
+        Ok(bytes_done)
+    }.await;
+
+    match result {
+        Err(e) => {
+            let _ = tokio::fs::remove_file(&part_path).await;
+            Err(e)
+        }
+        Ok(bytes_done) => {
+            let _ = app.emit("update:download:progress", UpdateDownloadProgress {
+                bytes: bytes_done,
+                total: Some(bytes_done),
+            });
+            tokio::fs::rename(&part_path, &dest_path)
+                .await
+                .map_err(|e| e.to_string())?;
+            Ok(dest_path.to_string_lossy().into_owned())
+        }
+    }
+}
+
+/// Fetches synced lyrics from Netease Cloud Music for a given artist + title.
+/// Performs a track search, then fetches the LRC string for the best match.
+/// Returns `None` if no match or no lyrics are found.
+#[tauri::command]
+pub async fn fetch_netease_lyrics(artist: String, title: String) -> Result<Option<String>, String> {
+    let client = reqwest::Client::builder()
+        .user_agent(subsonic_wire_user_agent())
+        .timeout(std::time::Duration::from_secs(8))
+        .build()
+        .map_err(|e| e.to_string())?;
+
+    let query = format!("{} {}", artist, title);
+    let params = [("s", query.as_str()), ("type", "1"), ("limit", "5")];
+    let search: serde_json::Value = client
+        .post("https://music.163.com/api/search/get")
+        .header("Referer", "https://music.163.com")
+        .form(&params)
+        .send()
+        .await
+        .map_err(|e| e.to_string())?
+        .json()
+        .await
+        .map_err(|e| e.to_string())?;
+
+    let song_id = match search["result"]["songs"][0]["id"].as_i64() {
+        Some(id) => id,
+        None => return Ok(None),
+    };
+
+    let lyrics: serde_json::Value = client
+        .get(format!(
+            "https://music.163.com/api/song/lyric?id={}&lv=1&kv=1&tv=-1",
+            song_id
+        ))
+        .header("Referer", "https://music.163.com")
+        .send()
+        .await
+        .map_err(|e| e.to_string())?
+        .json()
+        .await
+        .map_err(|e| e.to_string())?;
+
+    let lrc = lyrics["lrc"]["lyric"].as_str().unwrap_or("").trim().to_string();
+    Ok(if lrc.is_empty() { None } else { Some(lrc) })
+}
+
+/// Reads embedded synced / unsynced lyrics from a local audio file.
+///
+/// Priority order:
+///   MP3  → ID3v2 SYLT (synchronized, ms timestamps) → ID3v2 USLT (plain)
+///   FLAC → Vorbis SYNCEDLYRICS (LRC string)          → Vorbis LYRICS (plain)
+///
+/// Returns a standard LRC string (`[mm:ss.cc]line\n…`) for synced lyrics,
+/// or plain text for unsynced lyrics.  Returns `None` when no lyrics are found.
+/// Errors are silenced and mapped to `None` so the frontend falls through to the
+/// next lyrics source without crashing.
+#[tauri::command]
+pub fn get_embedded_lyrics(path: String) -> Option<String> {
+    use lofty::file::FileType;
+    use lofty::prelude::*;
+    use lofty::probe::Probe;
+
+    let fpath = std::path::Path::new(&path);
+    if !fpath.exists() {
+        return None;
+    }
+
+    // Detect file type from magic bytes only — no full tag read yet.
+    // guess_file_type() consumes self and returns Self, so reassign.
+    let probe = Probe::open(fpath).ok()?;
+    let probe = probe.guess_file_type().ok()?;
+    let file_type = probe.file_type();
+
+    // ── MP3 / MPEG: use the `id3` crate for SYLT / USLT ─────────────────────
+    // lofty's MpegFile::id3v2_tag field is pub — not accessible here.
+    // The `id3` crate exposes a clean public API for typed ID3v2 frames.
+    if matches!(file_type, Some(FileType::Mpeg)) {
+        use id3::{Content, Tag as Id3Tag};
+
+        if let Ok(tag) = Id3Tag::read_from_path(fpath) {
+            // 1. SYLT — millisecond-timestamped synced lyrics.
+            for frame in tag.frames() {
+                if frame.id() != "SYLT" {
+                    continue;
+                }
+                if let Content::SynchronisedLyrics(sylt) = frame.content() {
+                    // Only accept millisecond timestamps — MPEG-frame-based
+                    // timestamps can't be converted to wall-clock seconds.
+                    if sylt.timestamp_format != id3::frame::TimestampFormat::Ms {
+                        continue;
+                    }
+                    let lrc: String = sylt
+                        .content
+                        .iter()
+                        .filter_map(|(ms, text)| {
+                            let t = text.trim();
+                            if t.is_empty() {
+                                return None;
+                            }
+                            let mins = ms / 60_000;
+                            let secs = (ms % 60_000) / 1_000;
+                            let cs   = (ms % 1_000) / 10;
+                            // [mm:ss.cc] matches parseLrc's /\d+(?:\.\d*)?/ regex
+                            Some(format!("[{:02}:{:02}.{:02}]{}\n", mins, secs, cs, t))
+                        })
+                        .collect();
+                    if !lrc.is_empty() {
+                        return Some(lrc.trim_end().to_owned());
+                    }
+                }
+            }
+
+            // 2. USLT — unsynchronized lyrics, plain-text fallback.
+            for frame in tag.frames() {
+                if frame.id() != "USLT" {
+                    continue;
+                }
+                if let Content::Lyrics(uslt) = frame.content() {
+                    let text = uslt.text.trim();
+                    if !text.is_empty() {
+                        return Some(text.to_owned());
+                    }
+                }
+            }
+        }
+        return None; // MPEG file but no usable lyrics found
+    }
+
+    // ── FLAC / Vorbis / Opus / M4A: generic lofty tag API ────────────────────
+    // Vorbis SYNCEDLYRICS stores a complete LRC string in a plain comment field.
+    // In newer lofty versions, construct dynamic keys via ItemKey::from_key.
+    let tagged = probe.read().ok()?;
+    for tag in tagged.tags() {
+        if let Some(sync_key) = ItemKey::from_key(tag.tag_type(), "SYNCEDLYRICS") {
+            if let Some(lrc) = tag.get_string(sync_key) {
+                let lrc = lrc.trim();
+                if !lrc.is_empty() {
+                    return Some(lrc.to_owned());
+                }
+            }
+        }
+        if let Some(plain) = tag.get_string(ItemKey::Lyrics) {
+            let plain = plain.trim();
+            if !plain.is_empty() {
+                return Some(plain.to_owned());
+            }
+        }
+    }
+
+    None
+}
+
+/// Opens a directory in the OS file manager (Explorer / Finder / Nautilus).
+/// Uses platform-specific process spawning — tauri-plugin-shell's open() only
+/// allows https:// URLs per the capability scope and fails silently for paths.
+#[tauri::command]
+pub fn open_folder(path: String) -> Result<(), String> {
+    #[cfg(target_os = "windows")]
+    {
+        std::process::Command::new("explorer.exe")
+            .arg(&path)
+            .spawn()
+            .map_err(|e| e.to_string())?;
+    }
+    #[cfg(target_os = "macos")]
+    {
+        std::process::Command::new("open")
+            .arg(&path)
+            .spawn()
+            .map_err(|e| e.to_string())?;
+    }
+    #[cfg(target_os = "linux")]
+    {
+        std::process::Command::new("xdg-open")
+            .arg(&path)
+            .spawn()
+            .map_err(|e| e.to_string())?;
+    }
+    Ok(())
+}
+
+/// Progress payload emitted to the frontend during a ZIP download.
+/// `total` is `None` when the server doesn't send a `Content-Length` header
+/// (Navidrome on-the-fly ZIPs).
+#[derive(Clone, serde::Serialize)]
+#[serde(rename_all = "camelCase")]
+pub struct ZipProgress {
+    id: String,
+    bytes: u64,
+    total: Option<u64>,
+}
+
+/// Downloads a server-generated ZIP (album/playlist) directly to disk via streaming.
+/// Emits `download:zip:progress` events every 500 ms so the frontend can show
+/// live MB-counter without holding any binary data in the WebView process.
+/// Returns the final destination path on success.
+#[tauri::command]
+pub async fn download_zip(
+    id: String,
+    url: String,
+    dest_path: String,
+    app: tauri::AppHandle,
+) -> Result<String, String> {
+    use futures_util::StreamExt;
+    use std::time::{Duration, Instant};
+    use tokio::io::AsyncWriteExt;
+
+    const EMIT_INTERVAL: Duration = Duration::from_millis(500);
+
+    let client = reqwest::Client::builder()
+        .user_agent(subsonic_wire_user_agent())
+        .connect_timeout(Duration::from_secs(30))
+        .timeout(Duration::from_secs(7200)) // up to 2 h for large on-the-fly ZIPs
+        .build()
+        .map_err(|e| e.to_string())?;
+
+    let response = client.get(&url).send().await.map_err(|e| e.to_string())?;
+    if !response.status().is_success() {
+        return Err(format!("HTTP {}", response.status().as_u16()));
+    }
+
+    let total = response.content_length(); // None for Navidrome on-the-fly ZIPs
+    let part_path = format!("{dest_path}.part");
+
+    // Stream to .part file; rename on success, delete on error.
+    let result: Result<u64, String> = async {
+        let mut file = tokio::fs::File::create(&part_path)
+            .await
+            .map_err(|e| e.to_string())?;
+
+        let mut bytes_done: u64 = 0;
+        let mut stream = response.bytes_stream();
+        let mut last_emit = Instant::now();
+
+        while let Some(chunk) = stream.next().await {
+            let chunk = chunk.map_err(|e| e.to_string())?;
+            file.write_all(&chunk).await.map_err(|e| e.to_string())?;
+            bytes_done += chunk.len() as u64;
+
+            if last_emit.elapsed() >= EMIT_INTERVAL {
+                let _ = app.emit("download:zip:progress", ZipProgress {
+                    id: id.clone(),
+                    bytes: bytes_done,
+                    total,
+                });
+                last_emit = Instant::now();
+            }
+        }
+        file.flush().await.map_err(|e| e.to_string())?;
+        Ok(bytes_done)
+    }.await;
+
+    match result {
+        Err(e) => {
+            let _ = tokio::fs::remove_file(&part_path).await;
+            Err(e)
+        }
+        Ok(bytes_done) => {
+            // Final emission so the frontend sees 100 % (or final MB count).
+            let _ = app.emit("download:zip:progress", ZipProgress {
+                id: id.clone(),
+                bytes: bytes_done,
+                total: Some(bytes_done),
+            });
+            tokio::fs::rename(&part_path, &dest_path)
+                .await
+                .map_err(|e| e.to_string())?;
+            Ok(dest_path)
+        }
+    }
+}
+
+#[derive(serde::Serialize)]
+#[serde(rename_all = "camelCase")]
+pub struct HotCacheDownloadResult {
+    pub path: String,
+    pub size: u64,
+}

src-tauri/crates/psysonic-syncfs/src/cache/fs_utils.rs

@@ -0,0 +1,53 @@
+use std::path::Path;
+
+/// Recursively sums the size of all files under `root`.
+/// Missing roots, unreadable directories, and unreadable files are silently skipped.
+pub fn dir_size_recursive(root: &Path) -> u64 {
+    if !root.exists() {
+        return 0;
+    }
+    let mut total: u64 = 0;
+    let mut stack = vec![root.to_path_buf()];
+    while let Some(dir) = stack.pop() {
+        let rd = match std::fs::read_dir(&dir) {
+            Ok(r) => r,
+            Err(_) => continue,
+        };
+        for entry in rd.flatten() {
+            let path = entry.path();
+            if path.is_dir() {
+                stack.push(path);
+            } else if let Ok(meta) = std::fs::metadata(&path) {
+                total += meta.len();
+            }
+        }
+    }
+    total
+}
+
+/// Walks upward from `start_dir`, removing each empty directory using `remove_dir`
+/// (never `remove_dir_all`). Stops as soon as a non-empty directory is hit, the
+/// boundary is reached, or removal fails.
+///
+/// `boundary` is never removed and is treated as a hard stop. If `start_dir` is
+/// not under `boundary`, the function is a no-op.
+pub fn prune_empty_dirs_up_to(start_dir: &Path, boundary: &Path) {
+    let mut current = Some(start_dir.to_path_buf());
+    while let Some(dir) = current {
+        if dir == boundary || !dir.starts_with(boundary) {
+            break;
+        }
+        match std::fs::read_dir(&dir) {
+            Ok(mut entries) => {
+                if entries.next().is_some() {
+                    break;
+                }
+                if std::fs::remove_dir(&dir).is_err() {
+                    break;
+                }
+                current = dir.parent().map(|p| p.to_path_buf());
+            }
+            Err(_) => break,
+        }
+    }
+}

src-tauri/crates/psysonic-syncfs/src/cache/hot.rs

@@ -0,0 +1,231 @@
+use psysonic_analysis::analysis_runtime::enqueue_analysis_seed;
+use psysonic_audio as audio;
+use psysonic_core::user_agent::subsonic_wire_user_agent;
+
+use crate::file_transfer::stream_to_file;
+use super::downloads::{resolve_hot_cache_root, HotCacheDownloadResult};
+use super::offline::enqueue_analysis_seed_from_file;
+
+#[tauri::command]
+pub async fn download_track_hot_cache(
+    track_id: String,
+    server_id: String,
+    url: String,
+    suffix: String,
+    custom_dir: Option<String>,
+    app: tauri::AppHandle,
+) -> Result<HotCacheDownloadResult, String> {
+    let root = resolve_hot_cache_root(custom_dir, &app)?;
+    let cache_dir = root.join(&server_id);
+
+    tokio::fs::create_dir_all(&cache_dir)
+        .await
+        .map_err(|e| e.to_string())?;
+
+    let file_path = cache_dir.join(format!("{}.{}", track_id, suffix));
+    let path_str = file_path.to_string_lossy().to_string();
+
+    if file_path.exists() {
+        let size = tokio::fs::metadata(&file_path)
+            .await
+            .map(|m| m.len())
+            .unwrap_or(0);
+        crate::app_deprintln!(
+            "[hot-cache] download disk_hit track_id={} server_id={} bytes={}",
+            track_id,
+            server_id,
+            size
+        );
+        // Disk hit: still seed analysis, but do not block the command (full-file read); the
+        // prefetch worker runs invokes sequentially.
+        let app_seed = app.clone();
+        let tid = track_id.clone();
+        let fp = file_path.clone();
+        tokio::spawn(async move {
+            enqueue_analysis_seed_from_file(&app_seed, &tid, &fp).await;
+        });
+        return Ok(HotCacheDownloadResult {
+            path: path_str,
+            size,
+        });
+    }
+
+    crate::app_deprintln!(
+        "[hot-cache] download http_start track_id={} server_id={}",
+        track_id,
+        server_id
+    );
+
+    let client = reqwest::Client::builder()
+        .user_agent(subsonic_wire_user_agent())
+        .timeout(std::time::Duration::from_secs(120))
+        .build()
+        .map_err(|e| e.to_string())?;
+
+    let response = client.get(&url).send().await.map_err(|e| e.to_string())?;
+    if !response.status().is_success() {
+        return Err(format!("HTTP {}", response.status().as_u16()));
+    }
+
+    // Stream directly to a .part file; rename on success to avoid partial files.
+    let part_path = file_path.with_extension(format!("{suffix}.part"));
+    if let Err(e) = stream_to_file(response, &part_path).await {
+        let _ = tokio::fs::remove_file(&part_path).await;
+        return Err(e);
+    }
+    tokio::fs::rename(&part_path, &file_path)
+        .await
+        .map_err(|e| e.to_string())?;
+
+    let app_seed = app.clone();
+    let tid = track_id.clone();
+    let fp = file_path.clone();
+    tokio::spawn(async move {
+        enqueue_analysis_seed_from_file(&app_seed, &tid, &fp).await;
+    });
+
+    let size = tokio::fs::metadata(&file_path)
+        .await
+        .map(|m| m.len())
+        .unwrap_or(0);
+    crate::app_deprintln!(
+        "[hot-cache] download http_done track_id={} server_id={} bytes={}",
+        track_id,
+        server_id,
+        size
+    );
+    Ok(HotCacheDownloadResult {
+        path: path_str,
+        size,
+    })
+}
+
+/// Promotes bytes captured by the manual streaming path into hot cache on disk.
+/// Returns `Ok(None)` when no completed stream cache is available for this URL.
+#[tauri::command]
+pub async fn promote_stream_cache_to_hot_cache(
+    track_id: String,
+    server_id: String,
+    url: String,
+    suffix: String,
+    custom_dir: Option<String>,
+    app: tauri::AppHandle,
+    state: tauri::State<'_, audio::AudioEngine>,
+) -> Result<Option<HotCacheDownloadResult>, String> {
+    let root = resolve_hot_cache_root(custom_dir, &app)?;
+    let cache_dir = root.join(&server_id);
+    tokio::fs::create_dir_all(&cache_dir)
+        .await
+        .map_err(|e| e.to_string())?;
+
+    let file_path = cache_dir.join(format!("{}.{}", track_id, suffix));
+    let path_str = file_path.to_string_lossy().to_string();
+
+    if file_path.exists() {
+        let size = tokio::fs::metadata(&file_path)
+            .await
+            .map(|m| m.len())
+            .unwrap_or(0);
+        crate::app_deprintln!(
+            "[hot-cache] promote disk_hit track_id={} server_id={} bytes={}",
+            track_id,
+            server_id,
+            size
+        );
+        let app_seed = app.clone();
+        let tid = track_id.clone();
+        let fp = file_path.clone();
+        tokio::spawn(async move {
+            enqueue_analysis_seed_from_file(&app_seed, &tid, &fp).await;
+        });
+        return Ok(Some(HotCacheDownloadResult { path: path_str, size }));
+    }
+
+    let bytes = match audio::take_stream_completed_for_url(&state, &url) {
+        Some(b) => b,
+        None => {
+            crate::app_deprintln!(
+                "[hot-cache] promote skip track_id={} reason=no_completed_stream_for_url",
+                track_id
+            );
+            return Ok(None);
+        }
+    };
+
+    let part_path = file_path.with_extension(format!("{suffix}.part"));
+    if let Err(e) = tokio::fs::write(&part_path, &bytes).await {
+        let _ = tokio::fs::remove_file(&part_path).await;
+        return Err(e.to_string());
+    }
+    tokio::fs::rename(&part_path, &file_path)
+        .await
+        .map_err(|e| e.to_string())?;
+
+    let _ = enqueue_analysis_seed(&app, &track_id, &bytes).await;
+
+    let size = tokio::fs::metadata(&file_path)
+        .await
+        .map(|m| m.len())
+        .unwrap_or(0);
+    crate::app_deprintln!(
+        "[hot-cache] promote from_stream track_id={} server_id={} bytes={}",
+        track_id,
+        server_id,
+        size
+    );
+    Ok(Some(HotCacheDownloadResult { path: path_str, size }))
+}
+
+#[tauri::command]
+pub async fn get_hot_cache_size(custom_dir: Option<String>, app: tauri::AppHandle) -> u64 {
+    resolve_hot_cache_root(custom_dir, &app)
+        .map(|root| super::fs_utils::dir_size_recursive(&root))
+        .unwrap_or(0)
+}
+
+#[tauri::command]
+pub async fn delete_hot_cache_track(
+    local_path: String,
+    custom_dir: Option<String>,
+    app: tauri::AppHandle,
+) -> Result<(), String> {
+    let file_path = std::path::PathBuf::from(&local_path);
+    let existed = file_path.exists();
+    if existed {
+        tokio::fs::remove_file(&file_path)
+            .await
+            .map_err(|e| e.to_string())?;
+    }
+    crate::app_deprintln!(
+        "[hot-cache] delete file existed={} path_suffix={}",
+        existed,
+        file_path
+            .file_name()
+            .and_then(|s| s.to_str())
+            .unwrap_or("?")
+    );
+
+    let boundary = resolve_hot_cache_root(custom_dir, &app)?;
+    if let Some(parent) = file_path.parent() {
+        super::fs_utils::prune_empty_dirs_up_to(parent, &boundary);
+    }
+
+    Ok(())
+}
+
+/// Removes the entire hot cache root (`psysonic-hot-cache` for the active location).
+#[tauri::command]
+pub async fn purge_hot_cache(custom_dir: Option<String>, app: tauri::AppHandle) -> Result<(), String> {
+    let root = resolve_hot_cache_root(custom_dir, &app)?;
+    if !root.exists() {
+        return Ok(());
+    }
+    tokio::fs::remove_dir_all(&root)
+        .await
+        .map_err(|e| e.to_string())?;
+    crate::app_deprintln!(
+        "[hot-cache] purge root={} status=ok",
+        root.to_string_lossy()
+    );
+    Ok(())
+}

src-tauri/crates/psysonic-syncfs/src/cache/mod.rs

@@ -0,0 +1,4 @@
+mod fs_utils;
+pub mod offline;
+pub mod downloads;
+pub mod hot;

src-tauri/crates/psysonic-syncfs/src/cache/offline.rs

@@ -0,0 +1,142 @@
+use tauri::Manager;
+
+use psysonic_analysis::analysis_cache;
+use psysonic_analysis::analysis_runtime::enqueue_analysis_seed;
+use crate::DownloadSemaphore;
+
+use crate::file_transfer::{finalize_streamed_download, subsonic_http_client};
+
+// ─── Offline Track Cache ──────────────────────────────────────────────────────
+
+pub async fn enqueue_analysis_seed_from_file(
+    app: &tauri::AppHandle,
+    track_id: &str,
+    file_path: &std::path::Path,
+) {
+    if let Some(cache) = app.try_state::<analysis_cache::AnalysisCache>() {
+        if cache.cpu_seed_redundant_for_track(track_id).unwrap_or(false) {
+            return;
+        }
+    }
+    let bytes = match tokio::fs::read(file_path).await {
+        Ok(v) => v,
+        Err(_) => return,
+    };
+    if bytes.is_empty() {
+        return;
+    }
+    let _ = enqueue_analysis_seed(app, track_id, &bytes).await;
+}
+
+/// Downloads a single track to the app's offline cache directory.
+/// Returns the absolute file path so TypeScript can store it and later
+/// construct a `psysonic-local://<path>` URL for the audio engine.
+#[tauri::command]
+pub async fn download_track_offline(
+    track_id: String,
+    server_id: String,
+    url: String,
+    suffix: String,
+    custom_dir: Option<String>,
+    dl_sem: tauri::State<'_, DownloadSemaphore>,
+    app: tauri::AppHandle,
+) -> Result<String, String> {
+    // Determine base cache directory.
+    let cache_dir = if let Some(ref cd) = custom_dir {
+        let base = std::path::PathBuf::from(cd);
+        // Check that the volume/directory is still accessible.
+        if !base.exists() {
+            return Err("VOLUME_NOT_FOUND".to_string());
+        }
+        base.join(&server_id)
+    } else {
+        app.path()
+            .app_data_dir()
+            .map_err(|e| e.to_string())?
+            .join("psysonic-offline")
+            .join(&server_id)
+    };
+
+    tokio::fs::create_dir_all(&cache_dir)
+        .await
+        .map_err(|e| e.to_string())?;
+
+    let file_path = cache_dir.join(format!("{}.{}", track_id, suffix));
+    let path_str = file_path.to_string_lossy().to_string();
+
+    // Already cached — skip re-download (no semaphore needed).
+    if file_path.exists() {
+        return Ok(path_str);
+    }
+
+    // Acquire a download slot. The permit is held for the duration of the HTTP transfer
+    // and released automatically when this function returns (success or error).
+    let _permit = dl_sem.acquire().await.map_err(|e| e.to_string())?;
+
+    let client = subsonic_http_client(std::time::Duration::from_secs(120))?;
+
+    let response = client.get(&url).send().await.map_err(|e| e.to_string())?;
+    if !response.status().is_success() {
+        return Err(format!("HTTP {}", response.status().as_u16()));
+    }
+
+    let part_path = file_path.with_extension(format!("{suffix}.part"));
+    finalize_streamed_download(response, &file_path, &part_path).await?;
+
+    enqueue_analysis_seed_from_file(&app, &track_id, &file_path).await;
+
+    Ok(path_str)
+}
+
+/// Returns the total size in bytes of all files in the offline cache directory (and optional custom dir).
+#[tauri::command]
+pub async fn get_offline_cache_size(custom_dir: Option<String>, app: tauri::AppHandle) -> u64 {
+    let default_dir = match app.path().app_data_dir() {
+        Ok(d) => d.join("psysonic-offline"),
+        Err(_) => return 0,
+    };
+    let mut total = super::fs_utils::dir_size_recursive(&default_dir);
+
+    if let Some(cd) = custom_dir {
+        let custom = std::path::PathBuf::from(cd);
+        if custom != std::path::PathBuf::from("") {
+            total += super::fs_utils::dir_size_recursive(&custom);
+        }
+    }
+    total
+}
+
+/// Removes a cached track from the offline cache. Accepts the full local path
+/// (stored in OfflineTrackMeta) so it works regardless of which directory was used.
+/// After deleting the file, empty parent directories up to (but not including)
+/// `base_dir` are pruned using `remove_dir` (never `remove_dir_all`).
+#[tauri::command]
+pub async fn delete_offline_track(
+    local_path: String,
+    base_dir: Option<String>,
+    app: tauri::AppHandle,
+) -> Result<(), String> {
+    let file_path = std::path::PathBuf::from(&local_path);
+    if file_path.exists() {
+        tokio::fs::remove_file(&file_path)
+            .await
+            .map_err(|e| e.to_string())?;
+    }
+
+    // Determine the safe boundary — never delete at or above this directory.
+    let boundary = if let Some(bd) = base_dir.filter(|s| !s.is_empty()) {
+        std::path::PathBuf::from(bd)
+    } else {
+        app.path()
+            .app_data_dir()
+            .map_err(|e| e.to_string())?
+            .join("psysonic-offline")
+    };
+
+    if let Some(parent) = file_path.parent() {
+        super::fs_utils::prune_empty_dirs_up_to(parent, &boundary);
+    }
+
+    Ok(())
+}
+

src-tauri/crates/psysonic-syncfs/src/file_transfer.rs

@@ -0,0 +1,57 @@
+use std::path::Path;
+use std::time::Duration;
+
+/// Build a reqwest client with the standard Subsonic UA and a single overall timeout.
+/// For flows that need separate connect + read timeouts (long-running update/zip
+/// downloads with progress events), build the client inline.
+pub fn subsonic_http_client(timeout: Duration) -> Result<reqwest::Client, String> {
+    reqwest::Client::builder()
+        .user_agent(psysonic_core::user_agent::subsonic_wire_user_agent())
+        .timeout(timeout)
+        .build()
+        .map_err(|e| e.to_string())
+}
+
+/// Streams an HTTP response body to `dest_path` in chunks. Never buffers the full
+/// file in memory — keeps RAM flat regardless of file size.
+pub async fn stream_to_file(
+    response: reqwest::Response,
+    dest_path: &Path,
+) -> Result<(), String> {
+    use futures_util::StreamExt;
+    use tokio::io::AsyncWriteExt;
+
+    let mut file = tokio::fs::File::create(dest_path)
+        .await
+        .map_err(|e| e.to_string())?;
+    let mut stream = response.bytes_stream();
+    while let Some(chunk) = stream.next().await {
+        let chunk = chunk.map_err(|e| e.to_string())?;
+        file.write_all(&chunk).await.map_err(|e| e.to_string())?;
+    }
+    file.flush().await.map_err(|e| e.to_string())?;
+    Ok(())
+}
+
+/// Streams `response` to `part_path`, then renames `part_path` → `dest_path`.
+/// On any failure the partial `.part` file is best-effort removed so it does
+/// not linger on disk. Caller must ensure `dest_path.parent()` exists.
+///
+/// Note vs. previous inline implementations: the offline/device single-track
+/// flows used to leave a `.part` orphan if the final rename failed. This helper
+/// always cleans up, matching the batch-sync flow that already did.
+pub async fn finalize_streamed_download(
+    response: reqwest::Response,
+    dest_path: &Path,
+    part_path: &Path,
+) -> Result<(), String> {
+    if let Err(e) = stream_to_file(response, part_path).await {
+        let _ = tokio::fs::remove_file(part_path).await;
+        return Err(e);
+    }
+    if let Err(e) = tokio::fs::rename(part_path, dest_path).await {
+        let _ = tokio::fs::remove_file(part_path).await;
+        return Err(e.to_string());
+    }
+    Ok(())
+}

src-tauri/crates/psysonic-syncfs/src/lib.rs

@@ -0,0 +1,28 @@
+//! `psysonic-syncfs` — offline / hot-cache, device sync, and the shared HTTP
+//! download helpers used by both.
+//!
+//! This crate hosts the Tauri commands that read/write the on-disk caches
+//! (`offline_*`, `hot_cache_*`) and that copy tracks to mounted USB / SD-card
+//! devices (`sync_*`).
+
+use std::collections::HashMap;
+use std::sync::atomic::AtomicBool;
+use std::sync::{Arc, Mutex, OnceLock};
+
+// Re-export logging facade so submodules can keep `crate::app_eprintln!()`.
+pub use psysonic_core::{app_deprintln, app_eprintln, logging};
+
+pub mod cache;
+pub mod file_transfer;
+pub mod sync;
+
+/// Shared semaphore that caps simultaneous `download_track_offline` executions.
+pub type DownloadSemaphore = Arc<tokio::sync::Semaphore>;
+
+/// Per-job cancellation flags for `sync_batch_to_device`.
+/// Each running sync registers an `Arc<AtomicBool>` here; `cancel_device_sync`
+/// flips it.
+pub fn sync_cancel_flags() -> &'static Mutex<HashMap<String, Arc<AtomicBool>>> {
+    static FLAGS: OnceLock<Mutex<HashMap<String, Arc<AtomicBool>>>> = OnceLock::new();
+    FLAGS.get_or_init(|| Mutex::new(HashMap::new()))
+}

src-tauri/crates/psysonic-syncfs/src/sync/batch.rs

@@ -0,0 +1,532 @@
+use std::sync::atomic::{AtomicBool, Ordering};
+use std::sync::Arc;
+
+use tauri::Emitter;
+
+use crate::sync_cancel_flags;
+
+use crate::file_transfer::{finalize_streamed_download, subsonic_http_client};
+use super::device::{
+    build_track_path, get_removable_drives, is_path_on_mounted_volume, SyncBatchResult,
+    TrackSyncInfo,
+};
+
+#[tauri::command]
+pub async fn list_device_dir_files(dir: String) -> Result<Vec<String>, String> {
+    let root = std::path::PathBuf::from(&dir);
+    if !root.exists() {
+        return Err("VOLUME_NOT_FOUND".to_string());
+    }
+    let mut files = Vec::new();
+    let mut stack = vec![root];
+    while let Some(current) = stack.pop() {
+        let mut rd = match tokio::fs::read_dir(&current).await {
+            Ok(r) => r,
+            Err(_) => continue,
+        };
+        while let Ok(Some(entry)) = rd.next_entry().await {
+            let path = entry.path();
+            // Skip hidden dirs (e.g. .Trash-1000, .Ventoy, .fseventsd)
+            let is_hidden = path.file_name()
+                .and_then(|n| n.to_str())
+                .map(|n| n.starts_with('.'))
+                .unwrap_or(false);
+            if is_hidden { continue; }
+            if path.is_dir() {
+                stack.push(path);
+            } else {
+                files.push(path.to_string_lossy().to_string());
+            }
+        }
+    }
+    Ok(files)
+}
+
+/// Deletes a file from the device and prunes empty parent directories
+/// (up to 2 levels: album folder, then artist folder).
+#[tauri::command]
+pub async fn delete_device_file(path: String) -> Result<(), String> {
+    let p = std::path::PathBuf::from(&path);
+    if p.exists() {
+        tokio::fs::remove_file(&p).await.map_err(|e| e.to_string())?;
+        prune_empty_parents(&p, 2).await;
+    }
+    Ok(())
+}
+
+/// Prune empty parent directories up to `levels` levels above `file_path`.
+pub async fn prune_empty_parents(file_path: &std::path::Path, levels: usize) {
+    let mut current = file_path.parent().map(|d| d.to_path_buf());
+    for _ in 0..levels {
+        let Some(dir) = current else { break };
+        let is_empty = std::fs::read_dir(&dir)
+            .map(|mut rd| rd.next().is_none())
+            .unwrap_or(false);
+        if is_empty {
+            let _ = tokio::fs::remove_dir(&dir).await;
+            current = dir.parent().map(|d| d.to_path_buf());
+        } else {
+            break;
+        }
+    }
+}
+
+#[derive(serde::Deserialize)]
+#[serde(rename_all = "camelCase")]
+pub struct SubsonicAuthPayload {
+    base_url: String,
+    u: String,
+    t: String,
+    s: String,
+    v: String,
+    c: String,
+    f: String,
+}
+
+#[derive(serde::Deserialize, Clone)]
+pub struct DeviceSyncSourcePayload {
+    #[serde(rename = "type")]
+    source_type: String,
+    id: String,
+    /// Playlist display name — only present for playlist sources, used when
+    /// computing the playlist-folder path on the device.
+    #[serde(default)]
+    name: Option<String>,
+}
+
+#[derive(serde::Serialize)]
+#[serde(rename_all = "camelCase")]
+pub struct SyncDeltaResult {
+    add_bytes: u64,
+    add_count: u32,
+    del_bytes: u64,
+    del_count: u32,
+    available_bytes: u64,
+    tracks: Vec<serde_json::Value>,
+}
+
+pub async fn fetch_subsonic_songs(
+    client: &reqwest::Client,
+    auth: &SubsonicAuthPayload,
+    endpoint: &str,
+    id: &str,
+) -> Result<Vec<serde_json::Value>, String> {
+    let url = format!("{}/{}", auth.base_url, endpoint);
+    let query = vec![
+        ("u", auth.u.as_str()),
+        ("t", auth.t.as_str()),
+        ("s", auth.s.as_str()),
+        ("v", auth.v.as_str()),
+        ("c", auth.c.as_str()),
+        ("f", auth.f.as_str()),
+        ("id", id),
+    ];
+    let res = client.get(&url).query(&query).send().await.map_err(|e| e.to_string())?;
+    let json: serde_json::Value = res.json().await.map_err(|e| e.to_string())?;
+    
+    let root = json.get("subsonic-response").ok_or("No subsonic-response".to_string())?;
+    let songs = if endpoint == "getAlbum.view" {
+        root.get("album").and_then(|a| a.get("song"))
+    } else if endpoint == "getPlaylist.view" {
+        root.get("playlist").and_then(|p| p.get("entry"))
+    } else {
+        None
+    };
+
+    if let Some(arr) = songs.and_then(|s| s.as_array()) {
+        return Ok(arr.clone());
+    } else if let Some(obj) = songs.and_then(|s| s.as_object()) {
+        return Ok(vec![serde_json::Value::Object(obj.clone())]);
+    }
+    Ok(vec![])
+}
+
+#[tauri::command]
+pub async fn calculate_sync_payload(
+    sources: Vec<DeviceSyncSourcePayload>,
+    deletion_ids: Vec<String>,
+    auth: SubsonicAuthPayload,
+    target_dir: String,
+) -> Result<SyncDeltaResult, String> {
+    let client = subsonic_http_client(std::time::Duration::from_secs(30))?;
+
+    let mut add_bytes = 0;
+    let mut add_count = 0;
+    let mut del_bytes = 0;
+    let mut del_count = 0;
+    
+    let mut sync_tracks = Vec::new();
+    let (mut del_sources, mut add_sources) = (Vec::new(), Vec::new());
+    for s in sources {
+        if deletion_ids.contains(&s.id) {
+            del_sources.push(s);
+        } else {
+            add_sources.push(s);
+        }
+    }
+    
+    let mut handles: Vec<(DeviceSyncSourcePayload, tokio::task::JoinHandle<Vec<serde_json::Value>>)> = Vec::new();
+    for source in add_sources {
+        let auth_clone = SubsonicAuthPayload {
+            base_url: auth.base_url.clone(), u: auth.u.clone(), t: auth.t.clone(), s: auth.s.clone(),
+            v: auth.v.clone(), c: auth.c.clone(), f: auth.f.clone(),
+        };
+        let cli = client.clone();
+        let source_snapshot = source.clone();
+        let handle = tokio::spawn(async move {
+            let mut res_tracks = Vec::new();
+            if source.source_type == "album" {
+                if let Ok(ts) = fetch_subsonic_songs(&cli, &auth_clone, "getAlbum.view", &source.id).await { res_tracks.extend(ts); }
+            } else if source.source_type == "playlist" {
+                if let Ok(ts) = fetch_subsonic_songs(&cli, &auth_clone, "getPlaylist.view", &source.id).await { res_tracks.extend(ts); }
+            } else if source.source_type == "artist" {
+                let url = format!("{}/getArtist.view", auth_clone.base_url);
+                let query = vec![("u", auth_clone.u.as_str()), ("t", auth_clone.t.as_str()), ("s", auth_clone.s.as_str()), ("v", auth_clone.v.as_str()), ("c", auth_clone.c.as_str()), ("f", auth_clone.f.as_str()), ("id", &source.id)];
+                if let Ok(re) = cli.get(&url).query(&query).send().await {
+                   if let Ok(js) = re.json::<serde_json::Value>().await {
+                       if let Some(root) = js.get("subsonic-response").and_then(|r| r.get("artist")).and_then(|a| a.get("album")) {
+                          let arr = root.as_array().map(|a| a.clone()).unwrap_or_else(|| {
+                              root.as_object().map(|o| vec![serde_json::Value::Object(o.clone())]).unwrap_or_else(|| vec![])
+                          });
+                          for al in arr {
+                              if let Some(aid) = al.get("id").and_then(|i| i.as_str()) {
+                                  if let Ok(ts) = fetch_subsonic_songs(&cli, &auth_clone, "getAlbum.view", aid).await {
+                                      res_tracks.extend(ts);
+                                  }
+                              }
+                          }
+                       }
+                   }
+                }
+            }
+            res_tracks
+        });
+        handles.push((source_snapshot, handle));
+    }
+
+    let mut del_handles = Vec::new();
+    for source in del_sources {
+        let auth_clone = SubsonicAuthPayload {
+            base_url: auth.base_url.clone(), u: auth.u.clone(), t: auth.t.clone(), s: auth.s.clone(),
+            v: auth.v.clone(), c: auth.c.clone(), f: auth.f.clone(),
+        };
+        let cli = client.clone();
+        del_handles.push(tokio::spawn(async move {
+            let mut res_tracks = Vec::new();
+            if source.source_type == "album" {
+                if let Ok(ts) = fetch_subsonic_songs(&cli, &auth_clone, "getAlbum.view", &source.id).await { res_tracks.extend(ts); }
+            } else if source.source_type == "playlist" {
+                if let Ok(ts) = fetch_subsonic_songs(&cli, &auth_clone, "getPlaylist.view", &source.id).await { res_tracks.extend(ts); }
+            }
+            res_tracks
+        }));
+    }
+
+    // Dedup key is (source_id, track_id) rather than just track_id — a track
+    // appearing in both an album and a playlist needs to end up on the device
+    // in both locations (album tree + playlist folder).
+    let mut seen_by_source: std::collections::HashSet<(String, String)> = std::collections::HashSet::new();
+    for (source, handle) in handles {
+        if let Ok(ts) = handle.await {
+            let is_playlist = source.source_type == "playlist";
+            let mut playlist_position: u32 = 0;
+            for track in ts {
+                if let Some(tid) = track.get("id").and_then(|i| i.as_str()) {
+                    let key = (source.id.clone(), tid.to_string());
+                    if seen_by_source.contains(&key) { continue; }
+                    seen_by_source.insert(key);
+                    if is_playlist { playlist_position += 1; }
+                    let pl_name = if is_playlist { source.name.clone() } else { None };
+                    let pl_idx  = if is_playlist { Some(playlist_position) } else { None };
+
+                    let already_exists = {
+                        let suffix = track.get("suffix").and_then(|s| s.as_str()).unwrap_or("mp3");
+                        let artist_raw = track.get("artist").and_then(|v| v.as_str()).unwrap_or("");
+                        let album_artist = track.get("albumArtist")
+                            .and_then(|v| v.as_str())
+                            .filter(|s| !s.trim().is_empty())
+                            .unwrap_or(artist_raw);
+                        let sync_info = TrackSyncInfo {
+                            id: tid.to_string(),
+                            url: String::new(),
+                            suffix: suffix.to_string(),
+                            artist: artist_raw.to_string(),
+                            album_artist: album_artist.to_string(),
+                            album: track.get("album").and_then(|v| v.as_str()).unwrap_or("").to_string(),
+                            title: track.get("title").and_then(|v| v.as_str()).unwrap_or("").to_string(),
+                            track_number: track.get("track").and_then(|v| v.as_u64()).map(|n| n as u32),
+                            duration: track.get("duration").and_then(|v| v.as_u64()).map(|n| n as u32),
+                            playlist_name: pl_name.clone(),
+                            playlist_index: pl_idx,
+                        };
+                        let relative = build_track_path(&sync_info);
+                        let file_name = format!("{}.{}", relative, suffix);
+                        std::path::Path::new(&target_dir).join(&file_name).exists()
+                    };
+                    if !already_exists {
+                        add_count += 1;
+                        let size = track.get("size").and_then(|s| s.as_u64()).unwrap_or_else(|| {
+                            track.get("duration").and_then(|d| d.as_u64()).unwrap_or(0) * 320_000 / 8
+                        });
+                        add_bytes += size;
+                        // Embed playlist context in the track JSON so the frontend
+                        // can pass it back to sync_batch_to_device without re-computing it.
+                        let mut track_with_ctx = track.clone();
+                        if let Some(obj) = track_with_ctx.as_object_mut() {
+                            if let Some(name) = &pl_name {
+                                obj.insert("_playlistName".to_string(), serde_json::Value::String(name.clone()));
+                            }
+                            if let Some(idx) = pl_idx {
+                                obj.insert("_playlistIndex".to_string(), serde_json::Value::Number(idx.into()));
+                            }
+                        }
+                        sync_tracks.push(track_with_ctx);
+                    }
+                }
+            }
+        }
+    }
+
+    for handle in del_handles {
+        if let Ok(ts) = handle.await {
+            for track in ts {
+                del_count += 1;
+                let size = track.get("size").and_then(|s| s.as_u64()).unwrap_or_else(|| {
+                    track.get("duration").and_then(|d| d.as_u64()).unwrap_or(0) * 320_000 / 8
+                });
+                del_bytes += size;
+            }
+        }
+    }
+    
+    let mut available_bytes = 0;
+    for drive in get_removable_drives() {
+        if target_dir.starts_with(&drive.mount_point) {
+            available_bytes = drive.available_space;
+            break;
+        }
+    }
+
+    Ok(SyncDeltaResult {
+        add_bytes, add_count, del_bytes, del_count, available_bytes, tracks: sync_tracks,
+    })
+}
+
+/// Signals a running `sync_batch_to_device` job to stop after its current tracks finish.
+#[tauri::command]
+pub fn cancel_device_sync(job_id: String, app: tauri::AppHandle) {
+    if let Ok(flags) = sync_cancel_flags().lock() {
+        if let Some(flag) = flags.get(&job_id) {
+            flag.store(true, Ordering::Relaxed);
+        }
+    }
+    let _ = app.emit("device:sync:cancelled", serde_json::json!({ "jobId": job_id }));
+}
+
+/// Downloads a batch of tracks to a USB/SD device with controlled concurrency.
+/// At most 2 parallel writes run simultaneously to prevent I/O choking on USB.
+/// Emits throttled `device:sync:progress` events (max once per 500ms) and a
+/// final `device:sync:complete` event with the summary.
+#[tauri::command]
+pub async fn sync_batch_to_device(
+    tracks: Vec<TrackSyncInfo>,
+    dest_dir: String,
+    job_id: String,
+    expected_bytes: u64,
+    app: tauri::AppHandle,
+) -> Result<SyncBatchResult, String> {
+    use std::sync::atomic::{AtomicU32, Ordering};
+    use std::time::{Duration, Instant};
+    use tokio::sync::Mutex;
+
+    let dest_root = std::path::PathBuf::from(&dest_dir);
+    if !dest_root.exists() {
+        return Err("VOLUME_NOT_FOUND".to_string());
+    }
+    // Safety: verify dest_dir is on an actual mounted volume, not the root FS.
+    // This catches the case where a USB drive was unmounted but the empty
+    // mount-point directory still exists — writing there fills the root partition.
+    if !is_path_on_mounted_volume(&dest_root) {
+        return Err("NOT_MOUNTED_VOLUME".to_string());
+    }
+
+    // Safety: Ensure target logic hasn't exceeded physical volume capacities securely stopping dead bytes natively.
+    let drives = get_removable_drives();
+    let dest_canon = dest_root.canonicalize().unwrap_or_else(|_| dest_root.clone());
+    let dest_str = dest_canon.to_string_lossy();
+    
+    for drive in drives {
+        if dest_str.starts_with(&drive.mount_point) {
+            // Buffer of ~10 MB padding boundary natively mapped
+            if expected_bytes > drive.available_space.saturating_sub(10_000_000) {
+                return Err(format!("NOT_ENOUGH_SPACE"));
+            }
+            break;
+        }
+    }
+
+    // Register a cancellation flag for this job.
+    let cancel_flag = Arc::new(AtomicBool::new(false));
+    if let Ok(mut flags) = sync_cancel_flags().lock() {
+        flags.insert(job_id.clone(), cancel_flag.clone());
+    }
+
+    // Shared reqwest client — reused across all downloads.
+    let client = subsonic_http_client(Duration::from_secs(300))?;
+
+    // Concurrency limiter: max 2 parallel USB writes.
+    let semaphore = std::sync::Arc::new(tokio::sync::Semaphore::new(2));
+
+    // Counters.
+    let done    = std::sync::Arc::new(AtomicU32::new(0));
+    let skipped = std::sync::Arc::new(AtomicU32::new(0));
+    let failed  = std::sync::Arc::new(AtomicU32::new(0));
+
+    // Throttled event emission (max once per 500ms).
+    let last_emit = std::sync::Arc::new(Mutex::new(Instant::now()));
+    let total = tracks.len() as u32;
+
+    let mut handles = Vec::with_capacity(tracks.len());
+
+    for track in tracks {
+        let sem = semaphore.clone();
+        let cli = client.clone();
+        let app2 = app.clone();
+        let job = job_id.clone();
+        let dest = dest_dir.clone();
+        let d = done.clone();
+        let s = skipped.clone();
+        let f = failed.clone();
+        let le = last_emit.clone();
+        let cancel = cancel_flag.clone();
+
+        handles.push(tokio::spawn(async move {
+            let _permit = sem.acquire().await.expect("semaphore closed");
+
+            // Bail out if cancelled while waiting in the semaphore queue.
+            if cancel.load(Ordering::Relaxed) { return; }
+
+            let relative = build_track_path(&track);
+            let file_name = format!("{}.{}", relative, track.suffix);
+            let dest_path = std::path::Path::new(&dest).join(&file_name);
+            let path_str = dest_path.to_string_lossy().to_string();
+
+            let status;
+            if dest_path.exists() {
+                s.fetch_add(1, Ordering::Relaxed);
+                status = "skipped";
+            } else {
+                // Ensure parent directories exist.
+                if let Some(parent) = dest_path.parent() {
+                    if let Err(e) = tokio::fs::create_dir_all(parent).await {
+                        f.fetch_add(1, Ordering::Relaxed);
+                        let _ = app2.emit("device:sync:progress", serde_json::json!({
+                            "jobId": job, "trackId": track.id, "status": "error",
+                            "error": e.to_string(),
+                        }));
+                        return;
+                    }
+                }
+
+                let response = match cli.get(&track.url).send().await {
+                    Ok(r) if r.status().is_success() => r,
+                    Ok(r) => {
+                        f.fetch_add(1, Ordering::Relaxed);
+                        let _ = app2.emit("device:sync:progress", serde_json::json!({
+                            "jobId": job, "trackId": track.id, "status": "error",
+                            "error": format!("HTTP {}", r.status().as_u16()),
+                        }));
+                        return;
+                    }
+                    Err(e) => {
+                        f.fetch_add(1, Ordering::Relaxed);
+                        let _ = app2.emit("device:sync:progress", serde_json::json!({
+                            "jobId": job, "trackId": track.id, "status": "error",
+                            "error": e.to_string(),
+                        }));
+                        return;
+                    }
+                };
+
+                let part_path = dest_path.with_extension(format!("{}.part", track.suffix));
+                if let Err(e) = finalize_streamed_download(response, &dest_path, &part_path).await {
+                    f.fetch_add(1, Ordering::Relaxed);
+                    let _ = app2.emit("device:sync:progress", serde_json::json!({
+                        "jobId": job, "trackId": track.id, "status": "error",
+                        "error": e,
+                    }));
+                    return;
+                }
+
+                d.fetch_add(1, Ordering::Relaxed);
+                status = "done";
+            }
+
+            // Throttled progress event — max once per 500ms.
+            let should_emit = {
+                let mut guard = le.lock().await;
+                if guard.elapsed() >= Duration::from_millis(500) {
+                    *guard = Instant::now();
+                    true
+                } else {
+                    false
+                }
+            };
+            if should_emit {
+                let _ = app2.emit("device:sync:progress", serde_json::json!({
+                    "jobId": job, "trackId": track.id, "status": status, "path": path_str,
+                    "done": d.load(Ordering::Relaxed),
+                    "skipped": s.load(Ordering::Relaxed),
+                    "failed": f.load(Ordering::Relaxed),
+                    "total": total,
+                }));
+            }
+        }));
+    }
+
+    // Wait for all tasks to complete.
+    for handle in handles {
+        let _ = handle.await;
+    }
+
+    // Clean up the cancellation flag.
+    let was_cancelled = cancel_flag.load(Ordering::Relaxed);
+    if let Ok(mut flags) = sync_cancel_flags().lock() {
+        flags.remove(&job_id);
+    }
+
+    let result = SyncBatchResult {
+        done:    done.load(Ordering::Relaxed),
+        skipped: skipped.load(Ordering::Relaxed),
+        failed:  failed.load(Ordering::Relaxed),
+    };
+
+    // Final event so the frontend always sees 100%.
+    let _ = app.emit("device:sync:complete", serde_json::json!({
+        "jobId": job_id,
+        "done": result.done,
+        "skipped": result.skipped,
+        "failed": result.failed,
+        "total": total,
+        "cancelled": was_cancelled,
+    }));
+
+    Ok(result)
+}
+
+/// Deletes multiple files from the device in one call and prunes empty parent
+/// directories. Returns the number of files successfully deleted.
+#[tauri::command]
+pub async fn delete_device_files(paths: Vec<String>) -> Result<u32, String> {
+    let mut deleted: u32 = 0;
+    for path in &paths {
+        let p = std::path::PathBuf::from(path);
+        if p.exists() {
+            if tokio::fs::remove_file(&p).await.is_ok() {
+                deleted += 1;
+                prune_empty_parents(&p, 2).await;
+            }
+        }
+    }
+    Ok(deleted)
+}

src-tauri/crates/psysonic-syncfs/src/sync/device.rs

@@ -0,0 +1,387 @@
+use tauri::Emitter;
+
+use crate::file_transfer::{finalize_streamed_download, subsonic_http_client};
+
+// ─── Device Sync ─────────────────────────────────────────────────────────────
+
+/// Information about a single mounted removable drive.
+#[derive(Clone, serde::Serialize)]
+pub struct RemovableDrive {
+    pub name: String,
+    pub mount_point: String,
+    pub available_space: u64,
+    pub total_space: u64,
+    pub file_system: String,
+    pub is_removable: bool,
+}
+
+/// Returns all currently mounted removable drives.
+/// On Linux these are typically USB sticks / SD cards under /media or /run/media.
+/// On macOS they appear under /Volumes. On Windows they are separate drive letters.
+#[tauri::command]
+pub fn get_removable_drives() -> Vec<RemovableDrive> {
+    use sysinfo::Disks;
+    let disks = Disks::new_with_refreshed_list();
+    disks
+        .list()
+        .iter()
+        .filter(|d| d.is_removable())
+        .map(|d| RemovableDrive {
+            name: d.name().to_string_lossy().to_string(),
+            mount_point: d.mount_point().to_string_lossy().to_string(),
+            available_space: d.available_space(),
+            total_space: d.total_space(),
+            file_system: d.file_system().to_string_lossy().to_string(),
+            is_removable: true,
+        })
+        .collect()
+}
+
+/// Writes a `psysonic-sync.json` manifest to the root of the target directory.
+/// The file records which sources (albums/playlists/artists) are synced to this
+/// device so that another machine can pick them up without relying on localStorage.
+#[tauri::command]
+pub fn write_device_manifest(dest_dir: String, sources: serde_json::Value) -> Result<(), String> {
+    let path = std::path::Path::new(&dest_dir).join("psysonic-sync.json");
+    // Manifest v2: fixed "{AlbumArtist}/{Album}/{TrackNum} - {Title}.{ext}" schema,
+    // no user-configurable filename template. Readers still accept v1 manifests.
+    let payload = serde_json::json!({
+        "version": 2,
+        "schema": "fixed-v1",
+        "sources": sources
+    });
+    let json = serde_json::to_string_pretty(&payload).map_err(|e| e.to_string())?;
+    std::fs::write(&path, json).map_err(|e| e.to_string())
+}
+
+/// Reads `psysonic-sync.json` from the target directory.
+/// Returns the parsed JSON value, or null if the file doesn't exist.
+#[tauri::command]
+pub fn read_device_manifest(dest_dir: String) -> Option<serde_json::Value> {
+    let path = std::path::Path::new(&dest_dir).join("psysonic-sync.json");
+    let content = std::fs::read_to_string(&path).ok()?;
+    serde_json::from_str(&content).ok()
+}
+
+/// Per-entry result for `rename_device_files`.
+#[derive(serde::Serialize)]
+pub struct RenameResult {
+    #[serde(rename = "oldPath")]
+    old_path: String,
+    #[serde(rename = "newPath")]
+    new_path: String,
+    ok: bool,
+    error: Option<String>,
+}
+
+/// Atomically renames files on the device from their old path to the new fixed-
+/// schema path. Intended for the migration flow when switching away from the
+/// user-configurable template. All paths are relative to `target_dir`.
+///
+/// After renaming, removes any directories left empty under `target_dir`
+/// (so stale `{OldArtist}/{OldAlbum}/` trees don't linger).
+///
+/// Returns a per-entry result so the UI can show which renames succeeded
+/// and which failed. Does not roll back on partial failure — each `fs::rename`
+/// is atomic, so nothing can be half-renamed.
+#[tauri::command]
+pub fn rename_device_files(
+    target_dir: String,
+    pairs: Vec<(String, String)>,
+) -> Result<Vec<RenameResult>, String> {
+    let root = std::path::PathBuf::from(&target_dir);
+    if !root.exists() {
+        return Err("VOLUME_NOT_FOUND".to_string());
+    }
+    if !is_path_on_mounted_volume(&root) {
+        return Err("NOT_MOUNTED_VOLUME".to_string());
+    }
+
+    let mut results = Vec::with_capacity(pairs.len());
+    for (old_rel, new_rel) in pairs {
+        let old_abs = root.join(&old_rel);
+        let new_abs = root.join(&new_rel);
+
+        let entry = if old_rel == new_rel {
+            // Nothing to do, count as success so the UI can show "already correct".
+            RenameResult { old_path: old_rel, new_path: new_rel, ok: true, error: None }
+        } else if !old_abs.exists() {
+            RenameResult {
+                old_path: old_rel, new_path: new_rel,
+                ok: false, error: Some("source not found".to_string()),
+            }
+        } else if new_abs.exists() {
+            RenameResult {
+                old_path: old_rel, new_path: new_rel,
+                ok: false, error: Some("target already exists".to_string()),
+            }
+        } else {
+            // Ensure target parent exists.
+            if let Some(parent) = new_abs.parent() {
+                if let Err(e) = std::fs::create_dir_all(parent) {
+                    results.push(RenameResult {
+                        old_path: old_rel, new_path: new_rel,
+                        ok: false, error: Some(format!("mkdir: {}", e)),
+                    });
+                    continue;
+                }
+            }
+            match std::fs::rename(&old_abs, &new_abs) {
+                Ok(_) => RenameResult { old_path: old_rel, new_path: new_rel, ok: true, error: None },
+                Err(e) => RenameResult {
+                    old_path: old_rel, new_path: new_rel,
+                    ok: false, error: Some(e.to_string()),
+                },
+            }
+        };
+        results.push(entry);
+    }
+
+    // Clean up directories emptied by the renames. Walk depth-first and remove
+    // any dir whose only remaining contents were the files we moved out.
+    fn remove_empty_dirs(dir: &std::path::Path, root: &std::path::Path) {
+        if dir == root { return; }
+        let rd = match std::fs::read_dir(dir) {
+            Ok(r) => r,
+            Err(_) => return,
+        };
+        let mut empty = true;
+        let mut children: Vec<std::path::PathBuf> = Vec::new();
+        for entry in rd.flatten() {
+            let p = entry.path();
+            if p.is_dir() { children.push(p); } else { empty = false; }
+        }
+        for child in children {
+            remove_empty_dirs(&child, root);
+        }
+        // Re-check after recursion cleared subdirs.
+        let still_empty = std::fs::read_dir(dir).map(|r| r.count() == 0).unwrap_or(false);
+        if empty && still_empty {
+            let _ = std::fs::remove_dir(dir);
+        }
+    }
+    remove_empty_dirs(&root, &root);
+
+    Ok(results)
+}
+
+/// Writes an Extended-M3U playlist at `{dest_dir}/Playlists/{name}/{name}.m3u8`.
+/// References are sibling filenames (just `01 - Artist - Title.ext`) so the
+/// playlist is self-contained — moving/copying the folder anywhere keeps it
+/// working. Tracks are expected to be in playlist order (index starts at 1).
+#[tauri::command]
+pub fn write_playlist_m3u8(
+    dest_dir: String,
+    playlist_name: String,
+    tracks: Vec<TrackSyncInfo>,
+) -> Result<(), String> {
+    let safe_name = sanitize_or(&playlist_name, "Unnamed Playlist");
+    let playlist_dir = std::path::Path::new(&dest_dir).join("Playlists").join(&safe_name);
+    std::fs::create_dir_all(&playlist_dir).map_err(|e| e.to_string())?;
+    let file_path = playlist_dir.join(format!("{}.m3u8", safe_name));
+
+    let mut body = String::from("#EXTM3U\n");
+    for (i, track) in tracks.iter().enumerate() {
+        let idx = (i as u32) + 1;
+        let duration = track.duration.map(|d| d as i64).unwrap_or(-1);
+        let display_artist = if track.artist.trim().is_empty() { &track.album_artist[..] } else { &track.artist[..] };
+        let title = track.title.trim();
+        body.push_str(&format!("#EXTINF:{},{} - {}\n", duration, display_artist.trim(), title));
+        // Sibling filename — same shape as build_track_path's playlist branch.
+        let artist_safe = sanitize_or(display_artist, "Unknown Artist");
+        let title_safe  = sanitize_or(title,          "Unknown Title");
+        body.push_str(&format!("{:02} - {} - {}.{}\n", idx, artist_safe, title_safe, track.suffix));
+    }
+    std::fs::write(&file_path, body).map_err(|e| e.to_string())
+}
+
+/// Checks whether `path` sits on top of an active mount point (i.e. not the root
+/// filesystem). This prevents accidentally writing to `/media/usb` after the
+/// USB drive has been unmounted — at that point the path would fall through to `/`
+/// and fill the root partition.
+pub fn is_path_on_mounted_volume(path: &std::path::Path) -> bool {
+    use sysinfo::Disks;
+    let disks = Disks::new_with_refreshed_list();
+    let canonical = match path.canonicalize() {
+        Ok(c) => c,
+        Err(_) => return false, // path doesn't exist or isn't accessible
+    };
+    // On Windows, canonicalize() prepends "\\?\" (extended-path prefix).
+    // Strip it so that "\\?\E:\Music" compares correctly against mount point "E:\".
+    let canonical_raw = canonical.to_string_lossy().into_owned();
+    #[cfg(target_os = "windows")]
+    let canonical_str = canonical_raw.strip_prefix(r"\\?\").unwrap_or(&canonical_raw).to_string();
+    #[cfg(not(target_os = "windows"))]
+    let canonical_str = canonical_raw;
+    // Find the longest mount-point prefix that matches this path.
+    // Exclude the root "/" (or "C:\" on Windows) so we never "match" a fallback.
+    let mut best_len: usize = 0;
+    for disk in disks.list() {
+        let mp = disk.mount_point().to_string_lossy().to_string();
+        // Skip root mount points (Linux "/" and non-removable Windows drive roots like "C:\").
+        // Do NOT skip removable Windows drives (e.g. "E:\") — those are valid sync targets.
+        let is_windows_root = mp.len() == 3 && mp.ends_with(":\\") && !disk.is_removable();
+        if mp == "/" || is_windows_root {
+            continue;
+        }
+        if canonical_str.starts_with(&mp) && mp.len() > best_len {
+            best_len = mp.len();
+        }
+    }
+    best_len > 0
+}
+
+#[derive(serde::Deserialize, Clone)]
+pub struct TrackSyncInfo {
+    pub id: String,
+    pub url: String,
+    pub suffix: String,
+    /// Track artist — used in Extended M3U (#EXTINF) entries so playlists display
+    /// the actual performer rather than the album artist.
+    pub artist: String,
+    /// Album artist — used for the top-level folder so compilation albums stay together.
+    /// Falls back to `artist` in the frontend when the server has no albumArtist tag.
+    #[serde(rename = "albumArtist")]
+    pub album_artist: String,
+    pub album: String,
+    pub title: String,
+    #[serde(rename = "trackNumber")]
+    pub track_number: Option<u32>,
+    /// Duration in seconds — needed for Extended M3U (#EXTINF) playlist entries.
+    #[serde(default)]
+    pub duration: Option<u32>,
+    /// When set, the track belongs to a playlist source and is placed under
+    /// `Playlists/{name}/` with `playlist_index` as its filename prefix.
+    /// Same track synced from both an album and a playlist source ends up twice
+    /// on the device — once in the album tree, once in the playlist folder.
+    #[serde(default, rename = "playlistName")]
+    pub playlist_name: Option<String>,
+    #[serde(default, rename = "playlistIndex")]
+    pub playlist_index: Option<u32>,
+}
+
+/// Summary returned by `sync_batch_to_device` after all tracks are processed.
+#[derive(Clone, serde::Serialize)]
+pub struct SyncBatchResult {
+    pub done: u32,
+    pub skipped: u32,
+    pub failed: u32,
+}
+
+#[derive(serde::Serialize)]
+pub struct SyncTrackResult {
+    pub path: String,
+    pub skipped: bool,
+}
+
+/// Replaces characters that are invalid in file/directory names on Windows and
+/// most Unix filesystems with an underscore, and trims leading/trailing dots and
+/// spaces which cause issues on Windows. Underscore (not deletion) so that "AC/DC"
+/// and "ACDC" don't collapse into the same folder.
+pub fn sanitize_path_component(s: &str) -> String {
+    const INVALID: &[char] = &['/', '\\', ':', '*', '?', '"', '<', '>', '|'];
+    let sanitized: String = s
+        .chars()
+        .map(|c| if INVALID.contains(&c) || c.is_control() { '_' } else { c })
+        .collect();
+    sanitized.trim_matches(|c| c == '.' || c == ' ').to_string()
+}
+
+/// Sanitize and replace empty results with a placeholder — prevents paths like
+/// `//01 - .flac` when metadata is missing.
+pub fn sanitize_or(s: &str, fallback: &str) -> String {
+    let cleaned = sanitize_path_component(s);
+    if cleaned.is_empty() { fallback.to_string() } else { cleaned }
+}
+
+/// Builds the fixed device path for a track. When the track carries a playlist
+/// context it goes into the playlist folder, otherwise into the album tree.
+///
+/// Album-tree:  `{AlbumArtist}/{Album}/{TrackNum:02d} - {Title}.{ext}`
+/// Playlist:    `Playlists/{PlaylistName}/{PlaylistIndex:02d} - {Artist} - {Title}.{ext}`
+pub fn build_track_path(track: &TrackSyncInfo) -> String {
+    let relative = match (&track.playlist_name, track.playlist_index) {
+        (Some(name), Some(idx)) => {
+            let playlist = sanitize_or(name, "Unnamed Playlist");
+            let artist   = sanitize_or(&track.artist, "Unknown Artist");
+            let title    = sanitize_or(&track.title,  "Unknown Title");
+            format!("Playlists/{}/{:02} - {} - {}", playlist, idx, artist, title)
+        }
+        _ => {
+            let album_artist = sanitize_or(&track.album_artist, "Unknown Artist");
+            let album        = sanitize_or(&track.album,        "Unknown Album");
+            let title        = sanitize_or(&track.title,        "Unknown Title");
+            let track_num    = track.track_number.map(|n| format!("{:02}", n)).unwrap_or_else(|| "00".to_string());
+            format!("{}/{}/{} - {}", album_artist, album, track_num, title)
+        }
+    };
+    #[cfg(target_os = "windows")]
+    let relative = relative.replace('/', "\\");
+    relative
+}
+
+/// Downloads a single track to a USB/SD device using the configured filename template.
+/// Emits `device:sync:progress` events with `{ jobId, trackId, status, path? }`.
+#[tauri::command]
+pub async fn sync_track_to_device(
+    track: TrackSyncInfo,
+    dest_dir: String,
+    job_id: String,
+    app: tauri::AppHandle,
+) -> Result<SyncTrackResult, String> {
+    let relative = build_track_path(&track);
+    let file_name = format!("{}.{}", relative, track.suffix);
+    let dest_path = std::path::Path::new(&dest_dir).join(&file_name);
+    let path_str = dest_path.to_string_lossy().to_string();
+
+    if dest_path.exists() {
+        let _ = app.emit("device:sync:progress", serde_json::json!({
+            "jobId": job_id, "trackId": track.id, "status": "skipped", "path": path_str,
+        }));
+        return Ok(SyncTrackResult { path: path_str, skipped: true });
+    }
+
+    if let Some(parent) = dest_path.parent() {
+        tokio::fs::create_dir_all(parent)
+            .await
+            .map_err(|e| e.to_string())?;
+    }
+
+    let client = subsonic_http_client(std::time::Duration::from_secs(300))?;
+
+    let response = client.get(&track.url).send().await.map_err(|e| e.to_string())?;
+    if !response.status().is_success() {
+        let msg = format!("HTTP {}", response.status().as_u16());
+        let _ = app.emit("device:sync:progress", serde_json::json!({
+            "jobId": job_id, "trackId": track.id, "status": "error", "error": msg,
+        }));
+        return Err(msg);
+    }
+
+    let part_path = dest_path.with_extension(format!("{}.part", track.suffix));
+    if let Err(e) = finalize_streamed_download(response, &dest_path, &part_path).await {
+        let _ = app.emit("device:sync:progress", serde_json::json!({
+            "jobId": job_id, "trackId": track.id, "status": "error", "error": e,
+        }));
+        return Err(e);
+    }
+
+    let _ = app.emit("device:sync:progress", serde_json::json!({
+        "jobId": job_id, "trackId": track.id, "status": "done", "path": path_str,
+    }));
+    Ok(SyncTrackResult { path: path_str, skipped: false })
+}
+
+/// Computes the expected file paths for a batch of tracks under the fixed schema.
+/// Used by the cleanup flow to find orphans.
+#[tauri::command]
+pub fn compute_sync_paths(tracks: Vec<TrackSyncInfo>, dest_dir: String) -> Vec<String> {
+    tracks.iter().map(|track| {
+        let relative = build_track_path(track);
+        let file_name = format!("{}.{}", relative, track.suffix);
+        std::path::Path::new(&dest_dir)
+            .join(&file_name)
+            .to_string_lossy()
+            .to_string()
+    }).collect()
+}

src-tauri/crates/psysonic-syncfs/src/sync/mod.rs

@@ -0,0 +1,2 @@
+pub mod batch;
+pub mod device;