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>

package.json

@@ -13,7 +13,7 @@
     "test:watch": "vitest"
   },
   "dependencies": {
-    "animate.css": "^4.1.1",
+    "framer-motion": "^11.18.2",
     "lucide-react": "^0.460.0",
     "react": "^18.3.1",
     "react-dom": "^18.3.1",