fix(09-05): emit VolumeChanged event and persist state in ChangeVolume and MuteToggle

backend/player/player.go

@@ -682,6 +682,8 @@ func (p *Player) ChangeVolume(deltaVolume int) error {
 	defer p.mu.Unlock()
 
 	p.setVolumeLocked(p.getUserVolume() + UserVolume(deltaVolume))
+	p.emitVolumeChanged()
+	p.saveState()
 
 	return nil
 }
@@ -696,6 +698,7 @@ func (p *Player) MuteToggle() error {
 	defer p.mu.Unlock()
 
 	p.volume.Silent = !p.volume.Silent
+	p.emitVolumeChanged()
 	p.saveState()
 
 	return nil