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/ImageWithTextBubble.tsx

@@ -12,7 +12,12 @@ export function ImageWithTextBubble({ post }: { post: Post }) {
   if (!att) return null;
   return (
     <div className="flex flex-col">
-      <SingleImageFrame postId={post.id} attachment={att} text={text} />
+      <SingleImageFrame
+        postId={post.id}
+        attachment={att}
+        text={text}
+        showFilename={post.categorySlug === "official-assets"}
+      />
       {text ? (
         <CollapsibleText
           wrapperClassName="px-4 pt-3"