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/data/export_import/inventory_json_codec.dart

@@ -79,6 +79,8 @@ class InventoryJsonCodec {
             'originPlace': l.originPlace,
             'abundance': l.abundance?.name,
             'preservationFormat': l.preservationFormat?.name,
+            'priceAmount': l.priceAmount,
+            'priceCurrency': l.priceCurrency,
           },
       ],
       'vernacularNames': [
@@ -338,6 +340,8 @@ class InventoryJsonCodec {
             PreservationFormat.values,
             m['preservationFormat'],
           ),
+          priceAmount: (m['priceAmount'] as num?)?.toDouble(),
+          priceCurrency: m['priceCurrency'] as String?,
         );
       }),
       vernacularNames: _rows(root, 'vernacularNames', (m) {