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-tauri/src/lib.rs

@@ -1386,6 +1386,13 @@ pub fn run() {
                 }
             }
 
+            // ── Audio device-change watcher ───────────────────────────────
+            {
+                use tauri::Manager;
+                let engine = app.state::<audio::AudioEngine>();
+                audio::start_device_watcher(&engine, app.handle().clone());
+            }
+
             Ok(())
         })
         .on_window_event(|window, event| {