perf: 图片渐进加载,缩短首屏等待

- 流内单图改用缩略图(原图仅在灯箱按需加载),体积大幅减小
- BubbleImage 加 decoding=async + 加载完淡入(ark-img-fade),图片逐张出现
- 视频海报/文件预览/推荐卡/热门榜补 decoding=async、lazy
原图无缩略图时自动回退,无回归。

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/components/messageStream/BubbleImage.tsx

@@ -68,9 +68,13 @@ export function BubbleImage({
       src={current}
       alt=""
       loading={loading}
-      className={className}
+      decoding="async"
+      className={`ark-img-fade ${className ?? ""}`}
       onLoad={(e) => {
         const img = e.currentTarget;
+        // Fade each image in as soon as it loads, so they appear progressively
+        // instead of the page seeming to wait for everything.
+        img.classList.add("is-loaded");
         if (img.naturalWidth && img.naturalHeight)
           onNaturalSize?.(img.naturalWidth, img.naturalHeight);
       }}