feat(stream): surface source filename on official-assets cards

Image, album, and video bubbles in the official-assets category now
render the attachment filename as a bottom-left overlay so editors can
identify the source asset at a glance. Shared AttachmentFilenameLabel
component mirrors the AttachmentDownloadPill style, uses
filenameWithExtension so MIME-only attachments still get a sensible
label, and is pointer-events-none so it never blocks the bubble's tap
target.

src/components/messageStream/bubbles/ImageBubble.tsx

@@ -4,5 +4,11 @@ import { SingleImageFrame } from "./SingleImageFrame";
 export function ImageBubble({ post }: { post: Post }) {
   const att = post.attachments[0];
   if (!att) return null;
-  return <SingleImageFrame postId={post.id} attachment={att} />;
+  return (
+    <SingleImageFrame
+      postId={post.id}
+      attachment={att}
+      showFilename={post.categorySlug === "official-assets"}
+    />
+  );
 }