add Logout, OnLogout to server manager and hook up OnLogout callbacks

backend/playbackmanager.go

@@ -78,6 +78,10 @@ func NewPlaybackManager(ctx context.Context, s *ServerManager, p *player.Player)
 		pm.startPollTimePos()
 	})
 
+	s.OnLogout(func() {
+		pm.StopAndClearPlayQueue()
+	})
+
 	return pm
 }
 
@@ -204,6 +208,12 @@ func (p *PlaybackManager) RemoveTracksFromQueue(trackIdxs []int) {
 	}
 }
 
+// Stop playback and clear the play queue.
+func (p *PlaybackManager) StopAndClearPlayQueue() {
+	p.player.Stop()
+	p.player.ClearPlayQueue()
+}
+
 func (p *PlaybackManager) checkScrobble(playDur time.Duration) {
 	if len(p.playQueue) == 0 || p.nowPlayingIdx < 0 {
 		return