feat(block2): publish my seeds to the network (completes publish->discover)

The other half of the market: share your marked lots as offers.
- VarietyRepository.shareableLots(): non-private lots WITH their id (stable,
  updatable offer id) + the variety label. Private lots never leave the device.
- OffersCubit.publishLots(): maps each via OfferMapper and publishes, tagged
  with the coarse area + signed by the derived key; counts acceptances; no-op
  offline or with no area.
- Market screen: a 'share my seeds' action (online only) — publishes, then
  re-discovers to show them; 'mark some seeds first' when nothing's shared.
- i18n en/es/pt.

14 tests green (repo query, cubit publish, market UI); analyzer clean for new code.

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

@@ -344,6 +344,9 @@
     "serversLabel": "Endereço web",
     "save": "Guardar",
     "saved": "Guardado",
-    "wanted": "Procuro"
+    "wanted": "Procuro",
+    "shareMine": "Partilhar as minhas sementes",
+    "sharedCount": "Partilhadas {n} sementes",
+    "nothingToShare": "Marca primeiro algumas sementes para dar, trocar ou vender"
   }
 }