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/test/db/schema/schema.dart

@@ -14,6 +14,7 @@ import 'schema_v7.dart' as v7;
 import 'schema_v8.dart' as v8;
 import 'schema_v9.dart' as v9;
 import 'schema_v10.dart' as v10;
+import 'schema_v11.dart' as v11;
 
 class GeneratedHelper implements SchemaInstantiationHelper {
   @override
@@ -39,10 +40,12 @@ class GeneratedHelper implements SchemaInstantiationHelper {
         return v9.DatabaseAtV9(db);
       case 10:
         return v10.DatabaseAtV10(db);
+      case 11:
+        return v11.DatabaseAtV11(db);
       default:
         throw MissingSchemaException(version, versions);
     }
   }
 
-  static const versions = const [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
+  static const versions = const [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11];
 }