perf(player): 续看点加载与进度写盘改为并行且复用同一请求

进播放页与首次准备播放会各要一次续看点，同一次请求跑两遍还串在首帧前；
现在并发请求复用同一 Future，并与上一条进度写盘并行等待，少一个网络往返的起播延迟。
剧场网格海报按布局宽度限制解码尺寸，半屏卡片不再按原图解码。

lib/pages/discover/widgets/drama_grid.dart

@@ -7,6 +7,7 @@ import '../../../core/data/models/drama.dart';
 import '../../../core/formatting/count_format.dart';
 import '../../../core/theme/app_colors.dart';
 import '../../../core/theme/app_dimens.dart';
+import '../../../core/widgets/remote_image.dart';
 import '../../../l10n/app_localizations.dart';
 
 const int _columns = 2;
@@ -218,16 +219,28 @@ class _DiscoverCoverImage extends StatelessWidget {
   Widget build(BuildContext context) {
     final url = AppConfig.mediaUrl(path);
     if (url.isEmpty) return _placeholder();
-    return Image.network(
-      url,
-      width: double.infinity,
-      height: double.infinity,
-      fit: BoxFit.cover,
-      loadingBuilder: (context, child, progress) {
-        if (progress == null) return child;
-        return _placeholder();
+    return LayoutBuilder(
+      builder: (context, constraints) {
+        // 卡片只有半屏宽，按原图解码一张竖版海报就是数 MB，按布局宽度限一下。
+        final decodeWidth = constraints.hasBoundedWidth
+            ? resolveImageDecodePixels(
+                constraints.maxWidth,
+                MediaQuery.devicePixelRatioOf(context),
+              )
+            : null;
+        return Image.network(
+          url,
+          width: double.infinity,
+          height: double.infinity,
+          fit: BoxFit.cover,
+          cacheWidth: decodeWidth,
+          loadingBuilder: (context, child, progress) {
+            if (progress == null) return child;
+            return _placeholder();
+          },
+          errorBuilder: (_, _, _) => _placeholder(),
+        );
       },
-      errorBuilder: (_, _, _) => _placeholder(),
     );
   }