fix(player): 修正封面比例与首帧切换闪烁

lib/pages/player/widgets/player_opening_frame.dart

@@ -12,6 +12,9 @@ import '../../../core/theme/app_colors.dart';
 import '../../../core/widgets/remote_image.dart';
 import 'scrub_preview_card.dart';
 
+const double _posterAspectRatio = 3 / 4;
+const double _posterBlurSigma = 18;
+
 /// 封面立即可见，雪碧图首帧解码完成后再覆盖封面。
 Widget? playerOpeningFrame(String? thumbnailsUrl, {String? posterUrl}) {
   final url = thumbnailsUrl?.trim() ?? '';
@@ -82,11 +85,33 @@ class _PlayerOpeningFrameState extends State<PlayerOpeningFrame> {
       children: [
         const ColoredBox(color: AppColors.mediaBlack),
         if (posterUrl != null)
-          RemoteImage(
-            posterUrl,
-            key: const Key('player-opening-poster'),
-            fit: BoxFit.cover,
-            fallbackColor: AppColors.mediaBlack,
+          Positioned.fill(
+            child: ImageFiltered(
+              key: const Key('player-opening-poster-blur'),
+              imageFilter: ui.ImageFilter.blur(
+                sigmaX: _posterBlurSigma,
+                sigmaY: _posterBlurSigma,
+              ),
+              child: RemoteImage(
+                posterUrl,
+                key: const Key('player-opening-poster-backdrop'),
+                fit: BoxFit.cover,
+                fallbackColor: AppColors.mediaBlack,
+              ),
+            ),
+          ),
+        if (posterUrl != null)
+          Center(
+            child: AspectRatio(
+              key: const Key('player-opening-poster-stage'),
+              aspectRatio: _posterAspectRatio,
+              child: RemoteImage(
+                posterUrl,
+                key: const Key('player-opening-poster'),
+                fit: BoxFit.cover,
+                fallbackColor: AppColors.mediaBlack,
+              ),
+            ),
           ),
         if (frame != null)
           SpriteCropPreview(