ui(thumbnails): force preview images to fixed ratio

Use object-fill for compact preview thumbnails so rank-list covers and
file previews fill their fixed boxes without cropping or black bars. This
keeps the list layout stable while matching the desired compressed-ratio
thumbnail treatment.

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 bg-[#111116] object-contain"
+          className="h-16 w-16 shrink-0 rounded-lg object-fill"
         />
       ) : (
         <div