feat(media): 图片占位改为纯色块并去掉扫光

lib/pages/following/following_grid_card.dart

@@ -64,7 +64,6 @@ class FollowingGridCard extends StatelessWidget {
                     child: RemoteImage(
                       posterUrl,
                       fit: BoxFit.cover,
-                      showFallbackIcon: true,
                       fallbackColor: AppColors.contentSurface,
                     ),
                   ),

lib/pages/following/following_history_card.dart

@@ -75,7 +75,6 @@ class FollowingHistoryCard extends StatelessWidget {
               child: RemoteImage(
                 posterUrl,
                 fit: BoxFit.cover,
-                showFallbackIcon: true,
                 fallbackColor: AppColors.contentSurface,
               ),
             ),

lib/pages/following/widgets/continue_watching_banner.dart

@@ -40,7 +40,6 @@ class ContinueWatchingBanner extends StatelessWidget {
             RemoteImage(
               record.displayLandscapeCover,
               fit: BoxFit.cover,
-              showFallbackIcon: true,
               fallbackColor: AppColors.contentSurface,
             ),
             const DecoratedBox(