fix: comply with eBay size standardization — never submit placeholder or invented sizes

eBay's size standardization (enforced July 2026) blocks or holds apparel
and footwear listings whose Size is missing, non-standard, or a
placeholder like 'See description'. The app was writing 'See photos'
into Size whenever no tag was visible, and the publish pipeline could
backfill a missing Size with 'Regular' or an arbitrary allowed value.

- prompts.ts: size fallback is now a best standard size from visible
  measurements, or empty — never placeholder text
- publish.ts: drop Size:'Regular' default; size aspects are only filled
  from real listing data (buildAspects, freeTextDefault,
  reconcileAspects, addMissingAspects all guarded); placeholder size
  strings are stripped before submission
- ListingCard: Size is now editable in the review step, with a warning
  when an apparel/footwear listing has no size so the seller fills it
  in before posting
- lib/categories.ts: category sets shared between server pipeline and
  client UI

Fixes #20