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/variety_repository.dart

@@ -226,6 +226,8 @@ class ShareableLot extends Equatable {
     this.category,
     this.harvestYear,
     this.isOrganic = false,
+    this.priceAmount,
+    this.priceCurrency,
   });
 
   final String lotId;
@@ -246,9 +248,23 @@ class ShareableLot extends Equatable {
   /// filter the market for it.
   final bool isOrganic;
 
+  /// Asking price, only populated for [OfferStatus.sell] lots. Null amount on
+  /// a sale means "price to be agreed" — the offer publishes without a price.
+  final double? priceAmount;
+  final String? priceCurrency;
+
   @override
-  List<Object?> get props =>
-      [lotId, varietyId, summary, offerStatus, category, harvestYear, isOrganic];
+  List<Object?> get props => [
+    lotId,
+    varietyId,
+    summary,
+    offerStatus,
+    category,
+    harvestYear,
+    isOrganic,
+    priceAmount,
+    priceCurrency,
+  ];
 }
 
 /// One germination test on a lot; [rate] is derived (0..1), null when it can't
@@ -322,6 +338,8 @@ class VarietyLot extends Equatable {
     this.abundance,
     this.preservationFormat,
     this.offerStatus = OfferStatus.private,
+    this.priceAmount,
+    this.priceCurrency,
     this.germinationTests = const [],
     this.conditionChecks = const [],
   });
@@ -354,6 +372,11 @@ class VarietyLot extends Equatable {
   /// the network is Block 2.
   final OfferStatus offerStatus;
 
+  /// Asking price, meaningful when [offerStatus] is `sell`. Null amount means
+  /// "price to be agreed".
+  final double? priceAmount;
+  final String? priceCurrency;
+
   final List<GerminationEntry> germinationTests;
 
   /// Storage-condition checks, most-recent first (`conditionChecks.first` = last).
@@ -377,6 +400,8 @@ class VarietyLot extends Equatable {
     abundance,
     preservationFormat,
     offerStatus,
+    priceAmount,
+    priceCurrency,
     germinationTests,
     conditionChecks,
   ];
@@ -1281,9 +1306,12 @@ class VarietyRepository {
     Abundance? abundance,
     PreservationFormat? preservationFormat,
     OfferStatus offerStatus = OfferStatus.private,
+    double? priceAmount,
+    String? priceCurrency,
   }) async {
     final (created, updated) = await _stamp();
     final id = idGen.newId();
+    final forSale = offerStatus == OfferStatus.sell;
     await _db
         .into(_db.lots)
         .insert(
@@ -1306,6 +1334,8 @@ class VarietyRepository {
             abundance: Value(abundance),
             preservationFormat: Value(preservationFormat),
             offerStatus: Value(offerStatus),
+            priceAmount: Value(forSale ? priceAmount : null),
+            priceCurrency: Value(forSale ? priceCurrency : null),
           ),
         );
     return id;
@@ -1326,8 +1356,13 @@ class VarietyRepository {
     Abundance? abundance,
     PreservationFormat? preservationFormat,
     OfferStatus offerStatus = OfferStatus.private,
+    double? priceAmount,
+    String? priceCurrency,
   }) async {
     final (_, updated) = await _stamp();
+    // Price only makes sense on a lot offered for sale; clearing the sale
+    // status clears the price with it.
+    final forSale = offerStatus == OfferStatus.sell;
     await (_db.update(_db.lots)..where((l) => l.id.equals(lotId))).write(
       LotsCompanion(
         type: Value(type),
@@ -1343,6 +1378,8 @@ class VarietyRepository {
         abundance: Value(abundance),
         preservationFormat: Value(preservationFormat),
         offerStatus: Value(offerStatus),
+        priceAmount: Value(forSale ? priceAmount : null),
+        priceCurrency: Value(forSale ? priceCurrency : null),
         updatedAt: Value(updated),
         lastAuthor: Value(nodeId),
       ),
@@ -1580,6 +1617,12 @@ class VarietyRepository {
             category: v.category,
             harvestYear: l.harvestYear,
             isOrganic: v.isOrganic,
+            priceAmount: l.offerStatus == OfferStatus.sell
+                ? l.priceAmount
+                : null,
+            priceCurrency: l.offerStatus == OfferStatus.sell
+                ? l.priceCurrency
+                : null,
           ),
     ];
   }
@@ -2363,6 +2406,8 @@ class VarietyRepository {
       abundance: l.abundance,
       preservationFormat: l.preservationFormat,
       offerStatus: l.offerStatus,
+      priceAmount: l.priceAmount,
+      priceCurrency: l.priceCurrency,
       germinationTests: germinationTests,
       conditionChecks: conditionChecks,
       quantity: hasQuantity