feat: scroll to post bubble from recommended card + back-to-top button

Recommended cards already routed to /browse#post-<id>, but the stream had
no logic to scroll to the target bubble — and the post might not be paged
in yet. MessageStream now resolves the #post-<id> hash, auto-loads more
pages until the bubble renders, scrolls to it, and gives it a brief gold
highlight. Bubbles get scroll-mt so they clear the sticky header.

Also adds a global floating back-to-top button (BackToTop) mounted in
PublicLayout, shown after scrolling past 400px.

Bundles related staging UI work already present in the working tree.

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

src/components/messageStream/bubbles/AlbumBubble.tsx

@@ -10,6 +10,7 @@ import { autolink } from "../utils/autolink";
 import { downloadAttachment } from "../utils/downloadFile";
 import { formatBytes } from "../utils/formatBytes";
 import { postDisplayText } from "../utils/postText";
+import { useToast } from "../../Toast";
 
 const MAX_VISIBLE = 4;
 
@@ -32,14 +33,20 @@ function ImageListDownloadButton({
   attachment: Attachment;
 }) {
   const { t } = useI18n();
+  const { showToast } = useToast();
   const [isDownloading, setIsDownloading] = useState(false);
 
-  const handleDownload = () => {
+  const handleDownload = async () => {
     if (isDownloading) return;
     setIsDownloading(true);
-    void downloadAttachment(postId, attachment.id, attachment.filename)
-      .finally(() => setIsDownloading(false))
-      .catch(() => {});
+    try {
+      await downloadAttachment(postId, attachment.id, attachment.filename);
+      showToast(t("downloadOk"));
+    } catch {
+      showToast(t("downloadFail"), "error");
+    } finally {
+      setIsDownloading(false);
+    }
   };
 
   return (