feat(block2): publish my seeds to the network (completes publish->discover)

The other half of the market: share your marked lots as offers.
- VarietyRepository.shareableLots(): non-private lots WITH their id (stable,
  updatable offer id) + the variety label. Private lots never leave the device.
- OffersCubit.publishLots(): maps each via OfferMapper and publishes, tagged
  with the coarse area + signed by the derived key; counts acceptances; no-op
  offline or with no area.
- Market screen: a 'share my seeds' action (online only) — publishes, then
  re-discovers to show them; 'mark some seeds first' when nothing's shared.
- i18n en/es/pt.

14 tests green (repo query, cubit publish, market UI); analyzer clean for new code.

apps/app_seeds/test/data/shareable_lots_test.dart

@@ -0,0 +1,43 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:tane/db/database.dart';
+import 'package:tane/db/enums.dart';
+
+import '../support/test_support.dart';
+
+void main() {
+  late AppDatabase db;
+  setUp(() => db = newTestDatabase());
+  tearDown(() => db.close());
+
+  test('shareableLots returns non-private lots with their id and label',
+      () async {
+    final repo = newTestRepository(db);
+    final varietyId = await repo.addQuickVariety(
+      label: 'Tomate rosa',
+      category: 'Solanaceae',
+    );
+    final sharedId = await repo.addLot(
+      varietyId: varietyId,
+      offerStatus: OfferStatus.exchange,
+    );
+
+    final lots = await repo.shareableLots();
+    final shared = lots.where((l) => l.lotId == sharedId).toList();
+    expect(shared, hasLength(1));
+    expect(shared.single.summary, 'Tomate rosa');
+    expect(shared.single.offerStatus, OfferStatus.exchange);
+    expect(shared.single.category, 'Solanaceae');
+  });
+
+  test('private lots are never shareable', () async {
+    final repo = newTestRepository(db);
+    final varietyId = await repo.addQuickVariety(label: 'Pimiento');
+    final privateId = await repo.addLot(
+      varietyId: varietyId,
+      offerStatus: OfferStatus.private,
+    );
+
+    final lots = await repo.shareableLots();
+    expect(lots.where((l) => l.lotId == privateId), isEmpty);
+  });
+}