fix: restore SDL volume to MAX after device switch

Remove dead 'currentVolume' variable that was never written to, causing
SwitchDevice to always restore volume to 0 after switching audio devices.

- Remove currentVolume from package state
- SwitchDevice now sets mix.MAX_VOLUME after reopening device
- GetVolume128 now queries SDL directly instead of reading stale variable
- Fix parseSinkLine not stripping trailing ']' from [vol: X.XX] (caused
  volume to silently parse as 0 for sinks without MUTED tag)
- Fix flake.nix: make wayland/X11 buildInputs Linux-only for macOS compat
- Add unit tests for parseSinks, parseSinkLine, and ConvertVolume
- Add changelog.txt tracking all audio bug fixes

internal/mixer/mixer.go

@@ -23,10 +23,9 @@ var (
 	deviceOpen bool
 	deviceName string
 
-	currentMusic  *mix.Music
-	currentPath   string
-	currentLoops  int // 0 = once, -1 = forever
-	currentVolume int // 0..128, defaults to MAX_VOLUME
+	currentMusic *mix.Music
+	currentPath  string
+	currentLoops int // 0 = once, -1 = forever
 
 	// Config knobs (could be made public or read from your config)
 	defaultFreq      = 48000
@@ -351,6 +350,7 @@ func parseSinkLine(s string) (Sink, bool) {
 		volPart := rest[volStart:]
 		// "[vol: 0.65]" or "[vol: 0.65 MUTED]"
 		volPart = strings.TrimPrefix(volPart, "[vol:")
+		volPart = strings.TrimRight(volPart, "]")
 		volPart = strings.TrimSpace(volPart)
 		fields := strings.Fields(volPart)
 		if len(fields) > 0 {
@@ -411,11 +411,8 @@ func SwitchDevice(newName string) error {
 	wasPlaying := mix.PlayingMusic()
 	savedPath := currentPath
 	savedLoops := currentLoops
-	savedVolume := currentVolume
 
 	// Stop and release current music before reopening device
-	if wasPlaying {
-	}
 	mix.HaltMusic()
 	if currentMusic != nil {
 		currentMusic.Free()
@@ -441,8 +438,8 @@ func SwitchDevice(newName string) error {
 		return err
 	}
 
-	// Restore volume
-	mix.VolumeMusic(savedVolume)
+	// Restore volume to MAX after reopening device (matches Play() behavior)
+	mix.VolumeMusic(mix.MAX_VOLUME)
 
 	// Optionally resume previous track if there was one
 	if wasPlaying && savedPath != "" {
@@ -471,7 +468,7 @@ func SwitchDevice(newName string) error {
 func GetVolume128() int {
 	mu.Lock()
 	defer mu.Unlock()
-	return currentVolume
+	return mix.VolumeMusic(-1)
 }
 
 // Shutdown fully tears everything down. Call at app exit.