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/now_playing_screen.dart

@@ -21,6 +21,9 @@ class NowPlayingScreen extends StatelessWidget {
           icon: const Icon(Icons.keyboard_arrow_down),
           onPressed: () => Navigator.of(context).maybePop(),
         ),
+        actions: [
+          _SleepTimerButton(handler: handler),
+        ],
       ),
       body: SafeArea(
         child: StreamBuilder<MediaItem?>(
@@ -197,3 +200,75 @@ class _Controls extends StatelessWidget {
     );
   }
 }
+
+class _SleepTimerButton extends StatelessWidget {
+  const _SleepTimerButton({required this.handler});
+  final MeloAudioHandler handler;
+
+  void _showSleepOptions(BuildContext context) {
+    showModalBottomSheet(
+      context: context,
+      builder: (ctx) => Container(
+        color: Theme.of(ctx).scaffoldBackgroundColor,
+        child: Column(
+          mainAxisSize: MainAxisSize.min,
+          children: [
+            Padding(
+              padding: const EdgeInsets.all(16),
+              child: Text(
+                'Sleep-Timer',
+                style: Theme.of(ctx).textTheme.titleLarge,
+              ),
+            ),
+            ...const [5, 10, 15, 30, 60].map(
+              (minutes) => ListTile(
+                title: Text('$minutes Minuten'),
+                onTap: () {
+                  handler.sleepTimer.start(Duration(minutes: minutes));
+                  Navigator.pop(ctx);
+                },
+              ),
+            ),
+            ValueListenableBuilder<Duration?>(
+              valueListenable: handler.sleepTimer.remaining,
+              builder: (_, remaining, _) => remaining == null
+                  ? const SizedBox.shrink()
+                  : ListTile(
+                      title: const Text('Timer beenden'),
+                      leading: const Icon(Icons.close),
+                      onTap: () {
+                        handler.sleepTimer.cancel();
+                        Navigator.pop(ctx);
+                      },
+                    ),
+            ),
+            const SizedBox(height: 16),
+          ],
+        ),
+      ),
+    );
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return ValueListenableBuilder<Duration?>(
+      valueListenable: handler.sleepTimer.remaining,
+      builder: (context, remaining, _) {
+        if (remaining == null) {
+          return IconButton(
+            tooltip: 'Sleep-Timer',
+            icon: const Icon(Icons.bedtime_outlined),
+            onPressed: () => _showSleepOptions(context),
+          );
+        }
+        final minutes = remaining.inMinutes;
+        final seconds = (remaining.inSeconds % 60).toString().padLeft(2, '0');
+        return IconButton(
+          tooltip: 'Sleep-Timer: noch $minutes:$seconds',
+          icon: Icon(Icons.bedtime, color: MeloTheme.red),
+          onPressed: () => _showSleepOptions(context),
+        );
+      },
+    );
+  }
+}