fix(player): 使用雪碧图预热首帧兜底

test/player/scrub_preview_card_test.dart

@@ -67,13 +67,11 @@ void main() {
     await tester.pump();
 
     expect(loader.calls, 1);
-    expect(find.byKey(const Key('scrub-preview-poster')), findsOneWidget);
+    expect(find.byKey(const Key('scrub-preview-background')), findsOneWidget);
     expect(find.byKey(const Key('scrub-preview-sprite')), findsNothing);
   });
 
-  testWidgets('should_keep_poster_when_axis_and_local_frame_both_fail', (
-    tester,
-  ) async {
+  testWidgets('should_keep_background_when_frames_both_fail', (tester) async {
     final loader = _CountingThumbnailLoader();
     final localService = _localService(loader);
     final axis = _FakeAxisSource(axisReady: true);
@@ -90,7 +88,7 @@ void main() {
     await tester.pump();
 
     expect(loader.calls, 1);
-    expect(find.byKey(const Key('scrub-preview-poster')), findsOneWidget);
+    expect(find.byKey(const Key('scrub-preview-background')), findsOneWidget);
     expect(find.byKey(const Key('scrub-preview-sprite')), findsNothing);
     expect(find.byKey(const Key('scrub-preview-frame')), findsNothing);
   });
@@ -109,7 +107,6 @@ Widget _card({
           spriteAxisService: axis,
           thumbnailsUrl: thumbnailsUrl,
           episodeId: 'episode-1',
-          posterUrl: '',
           position: const Duration(seconds: 1),
           duration: const Duration(seconds: 10),
         ),