fix: resolve 4 critical audio mixer bugs (v0.5)

- SwitchDevice volume silently zeroed: Init uses cfg.SavedVolume,
  SwitchDevice restores saved volume after reopening device
- SwitchDevice not persisted to database: caller saves SelectedAudioDevice
- Device restore passes invalid 'system default' to SDL: now passes ''
- Stop() no-op when audio toggled off: removed AudioEnabled guard

Regression tests added for all 4 bugs. CI now runs go test before build.

internal/mixer/mixer.go

@@ -69,7 +69,7 @@ func Init(cfg *db.AppConfig, onUpdate func()) error {
 		return err
 	}
 
-	SetAllSinkVolumes(0.20)
+	SetAllSinkVolumes(cfg.SavedVolume)
 
 	inited = true
 	return nil
@@ -163,10 +163,6 @@ func Play(path string, loopForever bool, cfg *db.AppConfig) error {
 // Stop stops playback. If fadeMs > 0, fades out asynchronously; otherwise halts immediately.
 // It also frees the currently loaded music (so a new Play will reload).
 func Stop(fadeMs int, cfg *db.AppConfig) {
-	if !cfg.AudioEnabled {
-		return
-	}
-
 	mu.Lock()
 	defer mu.Unlock()
 	if !inited {
@@ -390,7 +386,7 @@ func CurrentDevice() string {
 }
 
 // SwitchDevice switches output devices, preserving volume and resuming playback of the current track/loop state if any.
-func SwitchDevice(newName string) error {
+func SwitchDevice(newName string, cfg *db.AppConfig) error {
 	mu.Lock()
 	defer mu.Unlock()
 
@@ -401,10 +397,7 @@ func SwitchDevice(newName string) error {
 	// Log device switch attempt
 	// Automatically defaults to "system default"
 	oldDevice := CurrentDevice() // Which device is to be closed
-	if oldDevice == "" {
-		oldDevice = "system default"
-	}
-	if newName == "" { // Which device is being switched to?
+	if newName == "" {          // Which device is being switched to?
 		newName = "system default"
 	}
 
@@ -431,6 +424,7 @@ func SwitchDevice(newName string) error {
 	// Attempt to open the newName device
 	if err := openDeviceLocked(newName); err != nil {
 		// If error switching to new device, switch back to the old device
+		// oldDevice is already "" for default (which openDeviceLocked expects)
 		if restoreErr := openDeviceLocked(oldDevice); restoreErr != nil {
 			// We should not hit an error when switching back, but if we do...
 			logger.Error("Failed to switch to device: %s", oldDevice)
@@ -439,8 +433,10 @@ func SwitchDevice(newName string) error {
 		return err
 	}
 
-	// Restore volume to MAX after reopening device (matches Play() behavior)
-	mix.VolumeMusic(mix.MAX_VOLUME)
+	// Restore saved volume after reopening device
+	if cfg != nil {
+		SetAllSinkVolumes(cfg.SavedVolume)
+	}
 
 	// Optionally resume previous track if there was one
 	if wasPlaying && savedPath != "" {