Use thumbnailUrl/posterUrl (and the image url itself for image-type
attachments) inside the 52x52 box of the file/document bubble, falling
back to the file-type icon when no preview is available.
Also tune the deep-link scroll-mt offset (82px / 98px) so a targeted
bubble lands just below the sticky header.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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>
