style: 热门榜封面与文件档预览缩略图改 object-contain 完整显示

热门榜单封面、文件档预览缩略图由 object-cover 改为 object-contain(文件预览
加底色),完整显示不裁剪。

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

src/components/messageStream/bubbles/FileDocBubble.tsx

@@ -47,7 +47,7 @@ function AttachmentRow({ postId, att }: { postId: string; att: Attachment }) {
           loading="lazy"
           decoding="async"
           onError={() => setPreviewFailed(true)}
-          className="h-16 w-16 shrink-0 rounded-lg object-cover"
+          className="h-16 w-16 shrink-0 rounded-lg bg-[#111116] object-contain"
         />
       ) : (
         <div