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/bubbles/FileDocBubble.tsx

@@ -45,6 +45,7 @@ function AttachmentRow({ postId, att }: { postId: string; att: Attachment }) {
           src={previewUrl}
           alt=""
           loading="lazy"
+          decoding="async"
           onError={() => setPreviewFailed(true)}
           className="h-16 w-16 shrink-0 rounded-lg object-cover"
         />