fix(audio): resume playback seamlessly on output device switch (#743) (#765)

* fix(audio): resume playback seamlessly on output device switch (#743)

When the OS default output device changed (Bluetooth, USB DAC, HDMI),
rodio/cpal had to reopen the stream, which silently stopped the active
sink and left the engine with no playback — causing the track to restart
from the beginning (or not restart at all after the null-payload bug).

Root cause (null-payload):
  audio_set_device emitted () (unit), which Tauri serialises to JSON
  null. The null-guard added for the "Rust handled replay internally"
  signal was therefore also triggered by manual device switches, so
  playTrack was never called and the engine stayed silent.
Fix: audio_set_device now emits the current playback position as f64
(null remains the exclusive "Rust handled" sentinel).

Rust-side seamless replay (device watcher path):
  reopen_output_stream captures a ResumeSnapshot before the blocking
  stream reopen, then calls try_resume_after_device_change, which:
    - local files (psysonic-local://): reopens the file, builds a new
      seekable source, seeks to the saved position — zero frontend
      round-trip, no audible restart.
    - fully-cached HTTP tracks (stream_completed_cache / spill file):
      replays from the in-memory or on-disk bytes — no re-download.
    - partial downloads / radio / paused: returns false → falls back to
      the existing frontend path (seekFallbackVisualTarget + playTrack).

Frontend (useAudioDeviceBridge):
  null payload  → Rust already resumed; skip playTrack.
  number payload → call playTrack + seekFallbackVisualTarget(position).

Visibility: pub(super) → pub(crate) on the play_input / progress_task
helpers that device_watcher.rs needs to call directly.

* refactor(audio): split device_resume module; add bridge tests

Split the 551-line device_watcher.rs (above the ~500-line soft ceiling)
by extracting ResumeSnapshot and try_resume_after_device_change into a
dedicated device_resume.rs module. device_watcher.rs is now 320 lines,
device_resume.rs 258 lines.

Add useAudioDeviceBridge.test.ts: 9 characterisation tests covering the
null-payload guard ("Rust replayed, skip playTrack"), the seek-fallback
path (position > 0.5 s sets seekFallbackVisualTarget), paused-device
branch (resetAudioPause), and the device-reset event path.

* chore(changelog): add entry for #765 (device switch seamless resume)

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

@@ -18,6 +18,7 @@ pub mod preload_commands;
 pub(crate) mod progress_task;
 pub mod radio_commands;
 pub mod transport_commands;
+mod device_resume;
 mod device_watcher;
 mod engine;
 #[cfg(any(target_os = "windows", target_os = "linux"))]