fix(music): mark only the active sleep timer preset selected

All timed presets shared one 'timed' bool, so arming any of 15/30/60
minutes checkmarked all three. The service now retains the armed
duration (sleepTimerDuration) and the menu marks the matching preset.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

test/services/music/music_playback_service_test.dart

@@ -929,6 +929,23 @@ void main() {
     });
   });
 
+  test('timed sleep timer exposes its armed duration for the preset menu', () async {
+    await h.playTracks([t1]);
+
+    h.service.setSleepTimer(const Duration(minutes: 30));
+    expect(h.service.sleepTimerActive, isTrue);
+    expect(h.service.sleepTimerDuration, const Duration(minutes: 30));
+    expect(h.service.sleepTimerEndOfTrack, isFalse);
+
+    h.service.setSleepTimer(null, endOfTrack: true);
+    expect(h.service.sleepTimerDuration, isNull);
+    expect(h.service.sleepTimerActive, isTrue);
+
+    h.service.setSleepTimer(null);
+    expect(h.service.sleepTimerActive, isFalse);
+    expect(h.service.sleepTimerDuration, isNull);
+  });
+
   test('end-of-track sleep timer suppresses arming and pauses at completion', () async {
     await h.playTracks([t1, t2]);
     expect(h.player.armed?.uri, _urlFor(t2));