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/variety_detail_cubit.dart

@@ -81,6 +81,8 @@ class VarietyDetailCubit extends Cubit<VarietyDetailState> {
     Abundance? abundance,
     PreservationFormat? preservationFormat,
     OfferStatus offerStatus = OfferStatus.private,
+    double? priceAmount,
+    String? priceCurrency,
   }) => _repo.addLot(
     varietyId: varietyId,
     type: type,
@@ -93,6 +95,8 @@ class VarietyDetailCubit extends Cubit<VarietyDetailState> {
     abundance: abundance,
     preservationFormat: preservationFormat,
     offerStatus: offerStatus,
+    priceAmount: priceAmount,
+    priceCurrency: priceCurrency,
   );
 
   Future<void> updateLot({
@@ -107,6 +111,8 @@ class VarietyDetailCubit extends Cubit<VarietyDetailState> {
     Abundance? abundance,
     PreservationFormat? preservationFormat,
     OfferStatus offerStatus = OfferStatus.private,
+    double? priceAmount,
+    String? priceCurrency,
   }) => _repo.updateLot(
     lotId: lotId,
     type: type,
@@ -119,6 +125,8 @@ class VarietyDetailCubit extends Cubit<VarietyDetailState> {
     abundance: abundance,
     preservationFormat: preservationFormat,
     offerStatus: offerStatus,
+    priceAmount: priceAmount,
+    priceCurrency: priceCurrency,
   );
 
   Future<void> deleteLot(String lotId) => _repo.softDeleteLot(lotId);