feat(media): 图片占位改为纯色块并去掉扫光

lib/pages/watch_records/widgets/watch_record_card.dart

@@ -70,9 +70,11 @@ class WatchRecordCard extends StatelessWidget {
                   key: ValueKey('watch-record-cover-${record.dramaId}'),
                   width: _coverWidth,
                   height: _coverHeight,
-                  showFallbackIcon: true,
-                  fallbackIconKey: ValueKey(
-                    'watch-record-cover-placeholder-${record.dramaId}',
+                  fallbackWidget: ColoredBox(
+                    key: ValueKey(
+                      'watch-record-cover-placeholder-${record.dramaId}',
+                    ),
+                    color: AppColors.raised,
                   ),
                 ),
               ),