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_en.g.dart

@@ -988,6 +988,12 @@ class Translations$share$en {
 	/// en: 'You have plenty — you could share some.'
 	String get nudge => 'You have plenty — you could share some.';
 
+	/// en: 'Price'
+	String get price => 'Price';
+
+	/// en: 'Leave it empty to agree it later'
+	String get priceHint => 'Leave it empty to agree it later';
+
 	/// en: 'Just for me'
 	String get private => 'Just for me';
 
@@ -2473,6 +2479,8 @@ extension on Translations {
 			'share.add' => 'Do you share it?',
 			'share.title' => 'Do you share it?',
 			'share.nudge' => 'You have plenty — you could share some.',
+			'share.price' => 'Price',
+			'share.priceHint' => 'Leave it empty to agree it later',
 			'share.private' => 'Just for me',
 			'share.gift' => 'To give away',
 			'share.exchange' => 'To swap',