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/AttachmentDownloadPill.tsx

@@ -5,6 +5,7 @@ import { useI18n } from "../../i18n";
 import type { Attachment } from "../../types/post";
 import { downloadAttachment } from "./utils/downloadFile";
 import { formatBytes } from "./utils/formatBytes";
+import { useToast } from "../Toast";
 
 type AttachmentDownloadPillProps = {
   postId: string;
@@ -20,15 +21,21 @@ export function AttachmentDownloadPill({
   className = "absolute left-2 top-2",
 }: AttachmentDownloadPillProps) {
   const { t } = useI18n();
+  const { showToast } = useToast();
   const [isDownloading, setIsDownloading] = useState(false);
 
-  const handleDownload = (e: MouseEvent<HTMLButtonElement>) => {
+  const handleDownload = async (e: MouseEvent<HTMLButtonElement>) => {
     e.stopPropagation();
     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 (