Match the inventory patterns the market was missing:
- Search: OffersState gains a `query` + `visibleOffers` getter and
OffersCubit.search() filters the already-discovered offers locally by
summary (case-insensitive), never re-hitting the transport. The filter
survives a refresh. Rounded search field mirrors inventory's.
- Pull-to-refresh: RefreshIndicator re-runs discovery for the current
area (empty state included, so a swipe re-scans). Text filter kept.
- i18n: market.searchHint / market.noMatches (en/es/pt).
Note: the shared slang output (strings*.g.dart, incl. the generated
Asturian locale) is regenerated wholesale, so it also carries the
in-progress `ast` locale keys already present in the working tree.
