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/Cargo.lock

@@ -3611,6 +3611,7 @@ dependencies = [
  "psysonic-analysis",
  "psysonic-audio",
  "psysonic-core",
+ "psysonic-syncfs",
  "reqwest",
  "ringbuf",
  "rodio",
@@ -3694,6 +3695,28 @@ dependencies = [
  "tauri",
 ]
 
+[[package]]
+name = "psysonic-syncfs"
+version = "1.46.0-dev"
+dependencies = [
+ "futures-util",
+ "id3",
+ "lofty",
+ "md5",
+ "psysonic-analysis",
+ "psysonic-audio",
+ "psysonic-core",
+ "reqwest",
+ "serde",
+ "serde_json",
+ "sysinfo",
+ "tauri",
+ "tauri-plugin-process",
+ "tauri-plugin-shell",
+ "tokio",
+ "url",
+]
+
 [[package]]
 name = "pxfm"
 version = "0.1.29"