fix: use pdsls.dev instead of broken atproto.at for URL fallback
atproto.at 404s on all records. pdsls.dev actually renders record
content for live PDS records. Also adds leaflet.pub/p/{did}/{rkey}
fallback for 208 leaflet docs without basePath (matching backend logic).
Affects ~1,619 docs (13% of atlas) that had no clickable link.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>