feat(offers): asking price on lots, published with sale offers (schema v11)

The data model always meant sale offers to carry an informational price
(data-model §2.8, sharing-model §4.4); with offer state collapsed onto
the Lot, the price lives there. Null amount on a sell lot = 'to be
agreed' — the NIP-99 event simply omits the price tag.

- schema v11: Lots.price_amount/price_currency, guarded migration,
  schema dump + generated schema_v11 for the round-trip test
- ShareableLot/VarietyLot carry the price (sell-only in shareableLots);
  addLot/updateLot persist it and clear it when the lot leaves sale
- OffersCubit.publishLots forwards it (outbox flush picks it up free)
- lot sheet: price + currency inputs revealed only on 'For sale', with
  the €/Ğ1/hours quick-picks extracted to a shared CurrencyQuickPicks
  widget (reused by the sale sheet)
- backup JSON round-trips the new columns
- i18n share.price/priceHint in en/es/pt/ast

apps/app_seeds/lib/state/offers_cubit.dart

@@ -284,6 +284,8 @@ class OffersCubit extends Cubit<OffersState> {
           areaGeohash: areaGeohash,
           category: lot.category,
           isOrganic: lot.isOrganic,
+          priceAmount: lot.priceAmount,
+          priceCurrency: lot.priceCurrency,
           imageUrl: await _coverThumbnail(lot.varietyId),
         );
         final result = await transport.publish(offer);