refactor(core): consolidate shared app foundations

test/media/playback_timeline_test.dart

@@ -0,0 +1,41 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:plezy/media/playback_timeline.dart';
+
+void main() {
+  test('seek detection uses one shared strict threshold', () {
+    final timeline = PlaybackTimeline();
+
+    expect(timeline.updatePosition(const Duration(seconds: 5)), isFalse);
+    expect(timeline.updatePosition(const Duration(seconds: 11)), isTrue);
+    expect(timeline.position, const Duration(seconds: 11));
+  });
+
+  test('watched threshold accepts the exact boundary', () {
+    final timeline = PlaybackTimeline(duration: const Duration(seconds: 100), watchedThreshold: 0.9);
+
+    timeline.updatePosition(const Duration(seconds: 90));
+
+    expect(timeline.watchedThresholdReached, isTrue);
+  });
+
+  test('unknown duration is not watched and reports zero progress', () {
+    final timeline = PlaybackTimeline(position: const Duration(seconds: 30));
+
+    expect(timeline.updateDuration(Duration.zero), isFalse);
+    expect(timeline.watchedThresholdReached, isFalse);
+    expect(timeline.progressPercent, 0);
+  });
+
+  test('progress is clamped and reset clears prior playback timing', () {
+    final timeline = PlaybackTimeline(position: const Duration(seconds: 120), duration: const Duration(seconds: 100));
+
+    expect(timeline.progressPercent, 100);
+
+    timeline.reset(watchedThreshold: 0.8);
+
+    expect(timeline.position, Duration.zero);
+    expect(timeline.duration, isNull);
+    expect(timeline.watchedThreshold, 0.8);
+    expect(timeline.watchedThresholdReached, isFalse);
+  });
+}