Fix Shake Detector not working when app in background (#40)

* feat: update shake detection settings to reduce cooldown and feedback options

* fix: shake detector not detecting in background

* enhance shake action handling to avoid unnecessary feedback

* disable shake detector when player not playing anything

* refactor: remove outdated TODO regarding shake detection optimization

* refactor: comment out notifyListeners call in restartTimer method for clarity

lib/features/sleep_timer/core/sleep_timer.dart

@@ -28,6 +28,9 @@ class SleepTimer {
   /// The timer that will pause the player
   Timer? timer;
 
+  /// is the sleep timer actively counting down
+  bool get isActive => timer != null && timer!.isActive;
+
   /// for internal use only
   /// when the timer was started
   DateTime? startedAt;