feat(mornye): cache motion covers and refine player and search

Save album motion videos in app storage during downloads and prefer offline playback with an aspect-aware layout. Add cache management for saved videos.

Center active lyrics on opening, expand lyrics during manual scrolling, and adjust landscape header spacing. Group search rows and skeletons with dividers and remove redundant result icons.

Validation: Flutter analysis clean; 723 tests passed, 4 skipped; iOS simulator build installed.

test/motion_header_banner_test.dart

@@ -8,12 +8,16 @@ class _VideoPlatform extends VideoPlayerPlatform {
   int playCalls = 0;
   bool looping = false;
   double volume = 1;
+  DataSource? source;
 
   @override
   Future<void> init() async {}
 
   @override
-  Future<int?> createWithOptions(VideoCreationOptions options) async => 1;
+  Future<int?> createWithOptions(VideoCreationOptions options) async {
+    source = options.dataSource;
+    return 1;
+  }
 
   @override
   Stream<VideoEvent> videoEventsFor(int playerId) => Stream.value(
@@ -58,6 +62,37 @@ class _VideoPlatform extends VideoPlayerPlatform {
 }
 
 void main() {
+  testWidgets(
+    'offline cover uses a silent looping file and reports its ratio',
+    (tester) async {
+      final previousPlatform = VideoPlayerPlatform.instance;
+      final platform = _VideoPlatform();
+      VideoPlayerPlatform.instance = platform;
+      addTearDown(() => VideoPlayerPlatform.instance = previousPlatform);
+      double? ratio;
+      await tester.pumpWidget(
+        MaterialApp(
+          home: SizedBox(
+            width: 320,
+            height: 180,
+            child: MotionHeaderBanner(
+              videoUrl: 'file:///app/motion_artwork/cover.mp4',
+              fallback: const ColoredBox(color: Colors.black),
+              onAspectRatioChanged: (value) => ratio = value,
+            ),
+          ),
+        ),
+      );
+      await tester.pump();
+      expect(platform.source?.sourceType, DataSourceType.file);
+      expect(platform.looping, isTrue);
+      expect(platform.volume, 0);
+      expect(platform.playing, isTrue);
+      expect(ratio, closeTo(320 / 180, 0.001));
+      await tester.pumpWidget(const SizedBox());
+    },
+  );
+
   testWidgets('collapsed headers pause video and visible headers resume', (
     tester,
   ) async {