feat(market): save others' offers as favorites (Wallapop-style)

Adds a Favorites feature: a heart on the offer detail saves another
person's listing to an encrypted, per-identity SavedOffersStore
(keystore JSON snapshot, no plaintext at rest). A new /favorites screen
(wired from the drawer) lists saved offers offline-first and, when a
relay is reachable, flags ones that are gone as "no longer available".
i18n en/es/pt/ast; store + screen + detail-heart tests.

apps/app_seeds/test/services/saved_offers_store_test.dart

@@ -0,0 +1,100 @@
+import 'package:commons_core/commons_core.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:tane/services/saved_offers_store.dart';
+
+import '../support/test_support.dart';
+
+Offer _offer(
+  String id, {
+  String author = 'aa',
+  OfferType type = OfferType.gift,
+  num? price,
+  String? currency,
+  String geohash = 'u09',
+}) => Offer(
+  id: id,
+  authorPubkeyHex: author,
+  summary: 'Tomate $id',
+  type: type,
+  approxGeohash: geohash,
+  priceAmount: price,
+  priceCurrency: currency,
+  isOrganic: true,
+);
+
+void main() {
+  late SavedOffersStore store;
+
+  setUp(() => store = SavedOffersStore(InMemorySecretStore()));
+
+  test('keyOf is the author:id coordinate', () {
+    expect(SavedOffersStore.keyOf(_offer('x', author: 'bb')), 'bb:x');
+  });
+
+  test('starts empty', () async {
+    expect(await store.list(), isEmpty);
+    expect(await store.isSaved('aa:x'), isFalse);
+  });
+
+  test('save then list round-trips every field', () async {
+    await store.save(
+      _offer('1', type: OfferType.sale, price: 3, currency: 'Ğ1'),
+      savedAt: 100,
+    );
+    final saved = await store.list();
+    expect(saved, hasLength(1));
+    final o = saved.single;
+    expect(o.id, '1');
+    expect(o.authorPubkeyHex, 'aa');
+    expect(o.type, OfferType.sale);
+    expect(o.priceAmount, 3);
+    expect(o.priceCurrency, 'Ğ1');
+    expect(o.isOrganic, isTrue);
+    expect(await store.isSaved('aa:1'), isTrue);
+  });
+
+  test('list is newest-first by savedAt', () async {
+    await store.save(_offer('old'), savedAt: 100);
+    await store.save(_offer('new'), savedAt: 200);
+    expect([for (final o in await store.list()) o.id], ['new', 'old']);
+  });
+
+  test('saving the same coordinate again de-duplicates', () async {
+    await store.save(_offer('1'), savedAt: 100);
+    await store.save(_offer('1'), savedAt: 200); // same author:id
+    expect(await store.list(), hasLength(1));
+  });
+
+  test('remove drops it', () async {
+    await store.save(_offer('1'), savedAt: 100);
+    await store.remove('aa:1');
+    expect(await store.list(), isEmpty);
+    expect(await store.isSaved('aa:1'), isFalse);
+  });
+
+  test('toggle saves then removes and reports the new state', () async {
+    expect(await store.toggle(_offer('1'), savedAt: 100), isTrue);
+    expect(await store.isSaved('aa:1'), isTrue);
+    expect(await store.toggle(_offer('1'), savedAt: 200), isFalse);
+    expect(await store.isSaved('aa:1'), isFalse);
+  });
+
+  test('scopes are isolated by account', () async {
+    final secret = InMemorySecretStore();
+    final a = SavedOffersStore(secret, accountScope: 'acct1');
+    final b = SavedOffersStore(secret, accountScope: 'acct2');
+    await a.save(_offer('1'), savedAt: 100);
+    expect(await a.list(), hasLength(1));
+    expect(await b.list(), isEmpty);
+  });
+
+  test('changes stream fires on save and remove', () async {
+    final events = <void>[];
+    final sub = store.changes.listen(events.add);
+    await store.save(_offer('1'), savedAt: 100);
+    await store.remove('aa:1');
+    await Future<void>.delayed(Duration.zero);
+    expect(events, hasLength(2));
+    await sub.cancel();
+  });
+}