fix: active device highlight incorrect when using default device

- mixer.Init: convert database 'default' to '' for SDL default device
- AudioOptions.Init: fallback to first SDL device when current device is default
  and config stores 'default'

Fixes known issue from changelog.txt

internal/mixer/mixer.go

@@ -64,7 +64,11 @@ func Init(cfg *db.AppConfig, onUpdate func()) error {
 		logger.Error("Failed to initialize MP3/OGG support: %v", err)
 	}
 
-	if err := openDeviceLocked(cfg.SelectedAudioDevice); err != nil {
+	deviceName := cfg.SelectedAudioDevice
+	if deviceName == "default" {
+		deviceName = ""
+	}
+	if err := openDeviceLocked(deviceName); err != nil {
 		sdl.QuitSubSystem(sdl.INIT_AUDIO)
 		return err
 	}