P0 fixes from UX_AUDIT_2026-10-09.md:
- useMarketplaceItem now filters admin-hidden listings and skips
showcase-only (not-for-sale) items, so a direct /listing/... URL cannot
surface removed or non-sale content.
- Listing detail hides Make an offer for logged-out users and for
fixed-price listings (Buy is the single primary CTA there).
Borrowed from the Plebeian market review: a /marketplace-guidelines page
(peer-to-peer, buyer-beware, no checkout, prohibited items, soft
moderation), linked from the marketplace hub; new marketTerms en/es keys.
Records the remaining borrowables in the audit doc.
