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/pages/Favorites/index.tsx

@@ -1,12 +1,13 @@
 import { Heart } from "lucide-react";
 import { Link } from "react-router-dom";
 import { useI18n } from "../../i18n";
+import { Reveal } from "../../motion";
 
 export default function Favorites() {
   const { t } = useI18n();
 
   return (
-    <div className="flex min-h-[60vh] flex-col items-center justify-center gap-5 px-4 py-12 text-center">
+    <Reveal className="flex min-h-[60vh] flex-col items-center justify-center gap-5 px-4 py-12 text-center">
       <div className="flex h-20 w-20 items-center justify-center rounded-full border border-ark-gold/30 bg-ark-gold/5">
         <Heart
           className="h-10 w-10 text-ark-gold/70"
@@ -33,6 +34,6 @@ export default function Favorites() {
       >
         {t("backToHome")}
       </Link>
-    </div>
+    </Reveal>
   );
 }