feat: 文档预览图改方形放大,下载胶囊整体放大

- FileDocBubble 预览缩略图 52px 圆形 → 64px 方形圆角(rounded-lg),行高自适应
- AttachmentDownloadPill 放大:图标框 24→30px、图标 14→18px、文字 11→12px

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

src/components/messageStream/bubbles/FileDocBubble.tsx

@@ -40,22 +40,22 @@ function AttachmentRow({ postId, att }: { postId: string; att: Attachment }) {
     att.thumbnailUrl ?? att.posterUrl ?? (isImage ? att.url : undefined);
 
   return (
-    <div className="group flex h-[52px] items-center gap-3">
+    <div className="group flex min-h-[64px] items-center gap-3">
       {previewUrl && !previewFailed ? (
         <img
           src={previewUrl}
           alt=""
           loading="lazy"
           onError={() => setPreviewFailed(true)}
-          className="h-[52px] w-[52px] shrink-0 rounded-full object-cover"
+          className="h-16 w-16 shrink-0 rounded-lg object-cover"
         />
       ) : (
         <div
-          className="flex h-[52px] w-[52px] shrink-0 items-center justify-center rounded-full"
+          className="flex h-16 w-16 shrink-0 items-center justify-center rounded-lg"
           style={{ backgroundColor: color }}
           aria-hidden="true"
         >
-          <Icon className="h-8 w-8 text-white" strokeWidth={2.1} />
+          <Icon className="h-9 w-9 text-white" strokeWidth={2.1} />
         </div>
       )}
       <div className="min-w-0 flex-1">