Make audio source switching instant with async initialization

When switching audio output source between HDMI and USB, the HDMI
audio device (hw:0,0) can take 18-31 seconds to initialize due to
hardware characteristics of the TC358743 chip. This caused the UI
to freeze during source changes.

Changes:
- Move startAudio() to background goroutine in SetAudioOutputSource
- Move SaveConfig() to background goroutine to avoid blocking on disk I/O
- Return immediately after updating in-memory config
- Audio will initialize in background while UI remains responsive

The in-memory config is updated synchronously so subsequent calls
see the new source immediately. Both async operations are protected
by their respective mutexes (audioMutex, configLock).

audio.go

@@ -278,14 +278,22 @@ func SetAudioOutputSource(source string) error {
 		return nil
 	}
 
-	stopOutputAudio()
 	config.AudioOutputSource = source
 
+	go func() {
+		stopOutputAudio()
 		if err := startAudio(); err != nil {
 			audioLogger.Error().Err(err).Str("source", source).Msg("Failed to start audio output after source change")
 		}
+	}()
 
-	return SaveConfig()
+	go func() {
+		if err := SaveConfig(); err != nil {
+			audioLogger.Error().Err(err).Msg("Failed to save config after audio source change")
+		}
+	}()
+
+	return nil
 }
 
 func RestartAudioOutput() error {