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/device_commands.rs

@@ -89,10 +89,19 @@ pub async fn audio_set_device(
 
     *state.stream_handle.lock().unwrap() = new_handle;
 
-    // Drop active sinks — they were bound to the old stream.
-    if let Some(s) = state.current.lock().unwrap().sink.take() { s.stop(); }
+    // Capture position and drop the active sink atomically so the position
+    // reading is still valid (play_started / paused_at intact before take).
+    let current_time = {
+        let mut cur = state.current.lock().unwrap();
+        let pos = cur.position();
+        if let Some(s) = cur.sink.take() { s.stop(); }
+        pos
+    };
     if let Some(s) = state.fading_out_sink.lock().unwrap().take() { s.stop(); }
 
-    app.emit("audio:device-changed", ()).map_err(|e| e.to_string())?;
+    // Emit the saved position so the frontend can use seekFallbackVisualTarget
+    // and resume from where the track was, rather than restarting from the beginning.
+    // null is reserved for "Rust already resumed internally" (see reopen_output_stream).
+    app.emit("audio:device-changed", current_time).map_err(|e| e.to_string())?;
     Ok(())
 }