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>

test/sleep_timer_test.dart

@@ -0,0 +1,118 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:melo/player/sleep_timer.dart';
+
+void main() {
+  group('SleepTimer with mocked clock', () {
+    late DateTime currentTime;
+    late SleepTimer sleepTimer;
+    late int elapsedCount;
+
+    setUp(() {
+      currentTime = DateTime(2024, 1, 1, 12, 0, 0);
+      elapsedCount = 0;
+      sleepTimer = SleepTimer(
+        onElapsed: () => elapsedCount++,
+        now: () => currentTime,
+      );
+    });
+
+    tearDown(() {
+      sleepTimer.cancel();
+    });
+
+    test('remaining is null when inactive', () {
+      expect(sleepTimer.remaining.value, isNull);
+    });
+
+    test('start sets remaining to duration', () {
+      sleepTimer.start(const Duration(minutes: 5));
+      expect(sleepTimer.remaining.value, isNotNull);
+      expect(sleepTimer.remaining.value!.inMinutes, equals(5));
+    });
+
+    test('remaining decrements as time passes', () {
+      sleepTimer.start(const Duration(minutes: 1));
+      expect(sleepTimer.remaining.value!.inSeconds, equals(60));
+
+      // Simulate 30 seconds passing
+      currentTime = currentTime.add(const Duration(seconds: 30));
+      sleepTimer.tick(); // Manually call tick to simulate timer
+
+      expect(sleepTimer.remaining.value!.inSeconds, equals(30));
+    });
+
+    test('fires callback when timer elapses', () {
+      sleepTimer.start(const Duration(minutes: 5));
+      expect(elapsedCount, equals(0));
+
+      // Simulate 5 minutes passing
+      currentTime = currentTime.add(const Duration(minutes: 5));
+      sleepTimer.tick();
+
+      expect(elapsedCount, equals(1), reason: 'onElapsed should fire once');
+      expect(sleepTimer.remaining.value, isNull);
+    });
+
+    test('cancel stops timer and clears remaining', () {
+      sleepTimer.start(const Duration(minutes: 5));
+      expect(sleepTimer.remaining.value, isNotNull);
+
+      sleepTimer.cancel();
+
+      expect(sleepTimer.remaining.value, isNull);
+      expect(elapsedCount, equals(0), reason: 'Should not fire after cancel');
+    });
+
+    test('does not fire if duration is zero', () {
+      sleepTimer.start(Duration.zero);
+
+      expect(elapsedCount, equals(0));
+      expect(sleepTimer.remaining.value, isNull);
+    });
+
+    test('does not fire if duration is negative', () {
+      sleepTimer.start(const Duration(seconds: -5));
+
+      expect(elapsedCount, equals(0));
+      expect(sleepTimer.remaining.value, isNull);
+    });
+
+    test('start while running replaces old timer', () {
+      sleepTimer.start(const Duration(minutes: 10));
+      final firstDuration = sleepTimer.remaining.value!;
+
+      sleepTimer.start(const Duration(minutes: 5));
+      final secondDuration = sleepTimer.remaining.value!;
+
+      expect(secondDuration < firstDuration, isTrue);
+      expect(elapsedCount, equals(0));
+    });
+
+    test('fires exactly once when elapsed', () {
+      sleepTimer.start(const Duration(minutes: 5));
+
+      // Advance time past deadline
+      currentTime = currentTime.add(const Duration(minutes: 6));
+      sleepTimer.tick();
+
+      expect(elapsedCount, equals(1));
+
+      // Even if we call tick again, should not fire twice
+      sleepTimer.tick();
+      expect(elapsedCount, equals(1), reason: 'Should fire only once');
+    });
+
+    test('does not fire if cancelled before elapse', () {
+      sleepTimer.start(const Duration(minutes: 5));
+
+      currentTime = currentTime.add(const Duration(minutes: 3));
+      sleepTimer.cancel();
+
+      currentTime = currentTime.add(const Duration(minutes: 3));
+      sleepTimer.tick();
+
+      expect(elapsedCount, equals(0));
+    });
+  });
+}