feat(audio): auto-switch to new audio output device at runtime

Adds a background device-watcher that polls the OS default output device
every 3 s via CPAL. When the device changes (Bluetooth headphones connecting,
USB DAC plugging in, etc.) the stream is reopened on the new device, the old
Sink is dropped (it was bound to the now-closed OutputStream), and
audio:device-changed is emitted to the frontend.

Frontend handler (TauriEventBridge): if playing, restarts the current track
from the saved position; if paused, clears the warm-pause flag so the next
resume uses the cold path (audio_play + seek) which creates a new Sink on
the new device.

Fixes #143 (audio through speakers after connecting Bluetooth headphones).
Also covers the intermittently reported single-channel output after long idle,
which can be caused by the OS reconfiguring the audio device in the background.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/App.tsx

@@ -413,6 +413,28 @@ function TauriEventBridge() {
     return () => { unlisten?.(); };
   }, []);
 
+  // Audio output device changed (Bluetooth headphones, USB DAC, etc.)
+  // The Rust device-watcher has already reopened the stream on the new device
+  // and dropped the old Sink, so we just need to restart playback.
+  useEffect(() => {
+    let unlisten: (() => void) | undefined;
+    listen('audio:device-changed', () => {
+      const { currentTrack, currentTime, isPlaying, playTrack, resetAudioPause } = usePlayerStore.getState();
+      if (!currentTrack) return;
+      if (isPlaying) {
+        const pos = currentTime;
+        const dur = currentTrack.duration || 1;
+        playTrack(currentTrack);
+        setTimeout(() => usePlayerStore.getState().seek(pos / dur), 600);
+      } else {
+        // Paused: clear warm-pause flag so the next resume uses the cold path
+        // (audio_play + seek) which creates a new Sink on the new device.
+        resetAudioPause();
+      }
+    }).then(u => { unlisten = u; });
+    return () => { unlisten?.(); };
+  }, []);
+
   // Sync tray-icon visibility with the user's stored setting.
   // Runs once on mount (initial sync) and again whenever the setting changes.
   const showTrayIcon = useAuthStore(s => s.showTrayIcon);