fix(audio): validate saved device exists in current enumeration

- Clear stale device IDs from DB when they don't exist in enumeration
- Add warning when device lookup fails in handleBackendDeviceChange
- Add logging to set_audio_output_device command for debugging

ALSA card numbers (hw:X,0) can change between boots when USB devices
are connected/disconnected. This fix prevents using invalid device IDs
that cause playback failures.

cc

src-tauri/src/config/audio_settings.rs

@@ -292,6 +292,7 @@ pub fn set_audio_output_device(
     state: tauri::State<'_, AudioSettingsState>,
     device: Option<String>,
 ) -> Result<(), String> {
+    log::info!("Command: set_audio_output_device {:?}", device);
     let store = state.store.lock().map_err(|e| format!("Lock error: {}", e))?;
     store.set_output_device(device.as_deref())
 }