feat: show attachment preview thumbnail in document bubble

Use thumbnailUrl/posterUrl (and the image url itself for image-type
attachments) inside the 52x52 box of the file/document bubble, falling
back to the file-type icon when no preview is available.

Also tune the deep-link scroll-mt offset (82px / 98px) so a targeted
bubble lands just below the sticky header.

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

src/components/messageStream/MessageBubble.tsx

@@ -35,7 +35,7 @@ export function MessageBubble({ post }: { post: Post }) {
   return (
     <div
       id={`post-${post.id}`}
-      className="mx-auto w-full max-w-[358px] scroll-mt-20 md:max-w-[680px] md:scroll-mt-24 lg:max-w-[900px] xl:max-w-[1120px]"
+      className="mx-auto w-full max-w-[358px] scroll-mt-[82px] md:max-w-[680px] md:scroll-mt-[98px] lg:max-w-[900px] xl:max-w-[1120px]"
     >
       <article
         className={`relative w-full overflow-hidden rounded-2xl bg-[#272632] text-left shadow-sm ${