feat(market): search + pull-to-refresh in the market list

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.

apps/app_seeds/lib/i18n/pt.i18n.json

@@ -40,6 +40,7 @@
     "langEs": "Español",
     "langEn": "English",
     "langPt": "Português",
+    "langAst": "Asturianu",
     "about": "Acerca de",
     "aboutText": "Inventário local e cifrado para sementes tradicionais. AGPL-3.0.",
     "aboutOpen": "Acerca do Tanemaki"
@@ -342,6 +343,8 @@
     "setAreaBody": "Diz ao mercado a tua zona aproximada para ver sementes por perto.",
     "searching": "A procurar pela tua zona…",
     "empty": "Ainda não há sementes partilhadas perto de ti",
+    "searchHint": "Procurar nestas sementes",
+    "noMatches": "Nenhuma semente partilhada corresponde à procura",
     "near": "Perto de ti",
     "contact": "Mensagem",
     "mine": "Tu",