Replace wpctl volume control with SDL_mixer VolumeMusic

Remove all PipeWire/wpctl system calls (SetDefaultSinkVolume,
listSinkIDs, listSinks, SetAllSinkVolumes, parseSinks, parseSinkLine,
Sink struct). Volume is now managed entirely through SDL_mixer's
VolumeMusic(), making audio fully system-agnostic.

Changes:
- mixer.go: delete ~170 lines of wpctl code; add SetVolume() public
  API using mix.VolumeMusic(math.Round(level * MAX_VOLUME))
- Init/Play/SwitchDevice set volume inline (they already hold the mutex)
- Play() now uses stored currentVolume instead of hardcoded MAX_VOLUME
- AudioOptions: SetAllSinkVolumes -> SetVolume
- mixer_test.go: add TestSetVolume (validation + error path coverage)

internal/page/AudioOptions.go

@@ -333,8 +333,8 @@ func (p *AudioOptions) UpdateVolume(level int) {
 	}
 	config.SavedVolume = newVol
 	p.Context.DbPtr.Save(&config)
-	if err := mixer.SetAllSinkVolumes(newVol); err != nil {
-		logger.Error("Error setting allSinkVolumes: %v", err)
+	if err := mixer.SetVolume(newVol); err != nil {
+		logger.Error("Error setting volume: %v", err)
 	}
 	if err := p.CreateVolumePercentItem(); err != nil {
 		logger.Error("failed to refresh volume percent item: %v", err)