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/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()))
+}