fix(player): 使用高清首帧并稳定切集占位

test/player/player_fullscreen_test.dart

@@ -1037,6 +1037,8 @@ void main() {
           accessible: false,
           accessType: PlayTicket.coinRequired,
           unlockCoin: 30,
+          posterUrl: '/files/locked-first-frame.jpg',
+          thumbnailsUrl: '/files/locked-thumbnails.vtt',
         ),
       ),
     );
@@ -1049,6 +1051,8 @@ void main() {
     expect(find.byKey(const Key('progress-inactive')), findsOneWidget);
     expect(find.byType(Slider), findsNothing);
     expect(find.byKey(const Key('player-single-placeholder')), findsNothing);
+    expect(find.byKey(const Key('player-opening-poster')), findsNothing);
+    expect(find.byKey(const Key('player-opening-sprite')), findsNothing);
     expect(find.byKey(const Key('player-video-crossfade')), findsNothing);
     expect(find.byKey(const Key('player-video-layer')), findsNothing);
     expect(
@@ -1487,6 +1491,8 @@ const _lockedDrama = Drama(
       isFree: false,
       locked: true,
       videoUrl: 'https://example.test/locked-demo-1.mp4',
+      posterUrl: '/files/locked-first-frame.jpg',
+      thumbnailsUrl: '/files/locked-thumbnails.vtt',
     ),
     Episode(
       id: 'locked-demo-2',