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/ui/variety_detail_screen.dart

@@ -16,6 +16,7 @@ import '../domain/seed_viability.dart';
 import '../domain/species_reference_links.dart';
 import '../i18n/strings.g.dart';
 import '../state/variety_detail_cubit.dart';
+import 'currency_quick_picks.dart';
 import 'harvest_date_picker.dart';
 import 'photo_pick.dart';
 import 'plantare_sheet.dart';
@@ -1464,6 +1465,18 @@ Future<void> _showLotSheet(
       existing?.originName != null || existing?.originPlace != null;
   var showAbundance = existing?.abundance != null;
   var showShare = selectedShare != OfferStatus.private;
+  final existingPrice = existing?.priceAmount;
+  final priceAmountCtrl = TextEditingController(
+    // Drop a trailing .0 so the field shows "5", not "5.0".
+    text: existingPrice == null
+        ? ''
+        : (existingPrice == existingPrice.roundToDouble()
+              ? existingPrice.toInt().toString()
+              : existingPrice.toString()),
+  );
+  final priceCurrencyCtrl = TextEditingController(
+    text: existing?.priceCurrency ?? '',
+  );
   return showModalBottomSheet<void>(
     context: context,
     isScrollControlled: true,
@@ -1660,6 +1673,49 @@ Future<void> _showLotSheet(
                             style: Theme.of(sheetContext).textTheme.bodySmall,
                           ),
                         ),
+                      // Price only when selling — informational, agreed and
+                      // paid off-app. Empty amount = "to be agreed".
+                      if (selectedShare == OfferStatus.sell) ...[
+                        const SizedBox(height: 12),
+                        Row(
+                          crossAxisAlignment: CrossAxisAlignment.start,
+                          children: [
+                            Expanded(
+                              child: TextField(
+                                key: const Key('lot.priceAmount'),
+                                controller: priceAmountCtrl,
+                                keyboardType:
+                                    const TextInputType.numberWithOptions(
+                                      decimal: true,
+                                    ),
+                                decoration: InputDecoration(
+                                  labelText: t.share.price,
+                                  helperText: t.share.priceHint,
+                                  border: const OutlineInputBorder(),
+                                ),
+                              ),
+                            ),
+                            const SizedBox(width: 12),
+                            Expanded(
+                              child: TextField(
+                                key: const Key('lot.priceCurrency'),
+                                controller: priceCurrencyCtrl,
+                                decoration: InputDecoration(
+                                  labelText: t.sale.currency,
+                                  border: const OutlineInputBorder(),
+                                ),
+                                onChanged: (_) => setState(() {}),
+                              ),
+                            ),
+                          ],
+                        ),
+                        const SizedBox(height: 8),
+                        CurrencyQuickPicks(
+                          controller: priceCurrencyCtrl,
+                          keyPrefix: 'lot',
+                          onChanged: () => setState(() {}),
+                        ),
+                      ],
                     ],
                     // Layer 2: advanced seed-bank details, collapsed by default.
                     // Only for seed lots, and (condition checks) an existing lot.
@@ -1710,6 +1766,10 @@ Future<void> _showLotSheet(
                   onPressed: () {
                     final originName = _nullIfBlank(originNameCtrl.text);
                     final originPlace = _nullIfBlank(originPlaceCtrl.text);
+                    final priceAmount = double.tryParse(
+                      priceAmountCtrl.text.replaceAll(',', '.').trim(),
+                    );
+                    final priceCurrency = _nullIfBlank(priceCurrencyCtrl.text);
                     if (editing) {
                       cubit.updateLot(
                         lotId: existing.id,
@@ -1723,6 +1783,8 @@ Future<void> _showLotSheet(
                         abundance: selectedAbundance,
                         preservationFormat: selectedPreservation,
                         offerStatus: selectedShare,
+                        priceAmount: priceAmount,
+                        priceCurrency: priceCurrency,
                       );
                     } else {
                       cubit.addLot(
@@ -1736,6 +1798,8 @@ Future<void> _showLotSheet(
                         abundance: selectedAbundance,
                         preservationFormat: selectedPreservation,
                         offerStatus: selectedShare,
+                        priceAmount: priceAmount,
+                        priceCurrency: priceCurrency,
                       );
                     }
                     Navigator.of(sheetContext).pop();