fix: detect platform from basePath, not lexicon
standard.site is a LEXICON, not a platform. platforms like pckt, leaflet,
and offprint use the standard.site lexicon.
- backend: migration detects pckt/leaflet from publication basePath
- frontend: shows 'standard.site' when platform unknown (not raw name)
- backfill-pds: also detects platform from basePath post-insert
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>