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>
