Add sleep timer feature

Timer stops playback (pause) after 5/10/15/30/60 min. Wall-clock
deadline is self-correcting against OS throttling; 1Hz tick drives
the countdown UI. Cancelled in stop() so no timer resurrects a paused
player after teardown.

- sleep_timer.dart: isolated time logic, injectable clock for testing
- audio_handler.dart: sleepTimer field + cancel() in stop()
- now_playing_screen.dart: AppBar button + bottom sheet
- 10 unit tests via mocked clock

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

lib/player/audio_handler.dart

@@ -1,12 +1,15 @@
 import 'package:audio_service/audio_service.dart';
 import 'package:just_audio/just_audio.dart';
+import 'sleep_timer.dart';
 
 /// Kern der Wiedergabe: kapselt just_audio hinter audio_service,
 /// damit Hintergrund-Wiedergabe + Lockscreen/Notification funktionieren.
 class MeloAudioHandler extends BaseAudioHandler with QueueHandler, SeekHandler {
   final AudioPlayer _player = AudioPlayer();
+  late final SleepTimer sleepTimer;
 
   MeloAudioHandler() {
+    sleepTimer = SleepTimer(onElapsed: pause);
     // just_audio-Events → audio_service PlaybackState
     _player.playbackEventStream.map(_transformEvent).pipe(playbackState);
 
@@ -46,6 +49,7 @@ class MeloAudioHandler extends BaseAudioHandler with QueueHandler, SeekHandler {
 
   @override
   Future<void> stop() async {
+    sleepTimer.cancel();
     await _player.stop();
     await super.stop();
   }