现阶段通知中心播放暂停切换功能已实现，但是上下一曲功能暂时消失，需要后续优化，这个commit是snapshot

lib/services/playback_service.dart

@@ -44,27 +44,27 @@ class PlaybackService {
   }
 
   // ---------- 控制 ----------
-  void pause() {
+  Future<void> pause() async {
     if (_initialized && _player != null) {
-      _player!.pause();
+      await _player!.pause();
     }
   }
 
-  void resume() {
+  Future<void> resume() async {
     if (_initialized && _player != null) {
-      _player!.play();
+      await _player!.play();
     }
   }
 
-  void stop() {
+  Future<void> stop() async {
     if (_initialized && _player != null) {
-      _player!.stop();
+      await _player!.stop();
     }
   }
 
-  void seek(Duration position) {
+  Future<void> seek(Duration position) async {
     if (_initialized && _player != null) {
-      _player!.seek(position);
+      await _player!.seek(position);
     }
   }