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/MessageBubble.tsx

@@ -35,7 +35,7 @@ export function MessageBubble({ post }: { post: Post }) {
   return (
     <div
       id={`post-${post.id}`}
-      className="mx-auto w-full max-w-[358px] md:max-w-[680px] lg:max-w-[900px] xl:max-w-[1120px]"
+      className="mx-auto w-full max-w-[358px] scroll-mt-20 md:max-w-[680px] md:scroll-mt-24 lg:max-w-[900px] xl:max-w-[1120px]"
     >
       <article
         className={`relative w-full overflow-hidden rounded-2xl bg-[#272632] text-left shadow-sm ${