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/i18n/strings_es.g.dart

@@ -547,6 +547,8 @@ class _Translations$share$es extends Translations$share$en {
 	@override String get add => '¿La compartes?';
 	@override String get title => '¿La compartes?';
 	@override String get nudge => 'Tienes de sobra: podrías compartir un poco.';
+	@override String get price => 'Precio';
+	@override String get priceHint => 'Déjalo vacío para acordarlo luego';
 	@override String get private => 'Solo para mí';
 	@override String get gift => 'Para regalar';
 	@override String get exchange => 'Para intercambiar';
@@ -1503,6 +1505,8 @@ extension on TranslationsEs {
 			'share.add' => '¿La compartes?',
 			'share.title' => '¿La compartes?',
 			'share.nudge' => 'Tienes de sobra: podrías compartir un poco.',
+			'share.price' => 'Precio',
+			'share.priceHint' => 'Déjalo vacío para acordarlo luego',
 			'share.private' => 'Solo para mí',
 			'share.gift' => 'Para regalar',
 			'share.exchange' => 'Para intercambiar',