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/ui/favorites_screen_test.dart

@@ -0,0 +1,77 @@
+import 'package:commons_core/commons_core.dart';
+import 'package:flutter/material.dart';
+import 'package:flutter_localizations/flutter_localizations.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:tane/i18n/strings.g.dart';
+import 'package:tane/services/saved_offers_store.dart';
+import 'package:tane/ui/favorites_screen.dart';
+
+import '../support/test_support.dart';
+
+Widget _wrap(Widget child) {
+  LocaleSettings.setLocaleSync(AppLocale.en);
+  return TranslationProvider(
+    child: MaterialApp(
+      locale: AppLocale.en.flutterLocale,
+      supportedLocales: AppLocaleUtils.supportedLocales,
+      localizationsDelegates: const [
+        GlobalMaterialLocalizations.delegate,
+        GlobalWidgetsLocalizations.delegate,
+        GlobalCupertinoLocalizations.delegate,
+      ],
+      home: child,
+    ),
+  );
+}
+
+Offer _offer(String id) => Offer(
+  id: id,
+  authorPubkeyHex: 'ab',
+  summary: 'Tomate $id',
+  type: OfferType.gift,
+  approxGeohash: 'u09',
+);
+
+void main() {
+  testWidgets('shows the empty state when nothing is saved', (tester) async {
+    final store = SavedOffersStore(InMemorySecretStore());
+    // No connection → no relay check, just the local snapshots.
+    await tester.pumpWidget(_wrap(FavoritesScreen(savedOffers: store)));
+    await tester.pumpAndSettle();
+
+    expect(
+      find.text('No favorites yet. Save offers you like from the market.'),
+      findsOneWidget,
+    );
+  });
+
+  testWidgets('lists saved offers newest-first', (tester) async {
+    final store = SavedOffersStore(InMemorySecretStore());
+    await store.save(_offer('old'), savedAt: 100);
+    await store.save(_offer('new'), savedAt: 200);
+
+    await tester.pumpWidget(_wrap(FavoritesScreen(savedOffers: store)));
+    await tester.pumpAndSettle();
+
+    expect(find.text('Tomate new'), findsOneWidget);
+    expect(find.text('Tomate old'), findsOneWidget);
+    final newY = tester.getTopLeft(find.text('Tomate new')).dy;
+    final oldY = tester.getTopLeft(find.text('Tomate old')).dy;
+    expect(newY, lessThan(oldY)); // newest on top
+  });
+
+  testWidgets('the heart removes a favorite', (tester) async {
+    final store = SavedOffersStore(InMemorySecretStore());
+    await store.save(_offer('1'), savedAt: 100);
+
+    await tester.pumpWidget(_wrap(FavoritesScreen(savedOffers: store)));
+    await tester.pumpAndSettle();
+    expect(find.text('Tomate 1'), findsOneWidget);
+
+    await tester.tap(find.byKey(const Key('favorites.remove.ab.1')));
+    await tester.pumpAndSettle();
+
+    expect(find.text('Tomate 1'), findsNothing);
+    expect(await store.isSaved('ab:1'), isFalse);
+  });
+}

apps/app_seeds/test/ui/market_offer_detail_screen_test.dart

@@ -3,6 +3,7 @@ import 'package:flutter/material.dart';
 import 'package:flutter_localizations/flutter_localizations.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:tane/i18n/strings.g.dart';
+import 'package:tane/services/saved_offers_store.dart';
 import 'package:tane/services/social_connection.dart';
 import 'package:tane/services/social_service.dart';
 import 'package:tane/services/social_settings.dart';
@@ -103,4 +104,50 @@ void main() {
 
     expect(find.byType(OfferHeroImage), findsNothing);
   });
+
+  testWidgets('the heart saves and unsaves a stranger\'s offer',
+      (tester) async {
+    final social = await SocialService.fromRootSeedHex('00' * 32);
+    final settings = SocialSettings(InMemorySecretStore());
+    await settings.setRelayUrls(const []);
+    final connection = SocialConnection(social: social, settings: settings);
+    final saved = SavedOffersStore(InMemorySecretStore());
+
+    await tester.pumpWidget(_wrap(MarketOfferDetailScreen(
+      offer: _offer(),
+      connection: connection,
+      savedOffers: saved,
+    )));
+    await tester.pumpAndSettle();
+
+    final heart = find.byKey(const Key('offerDetail.save'));
+    expect(heart, findsOneWidget);
+    expect(await saved.isSaved('${'ab' * 32}:lot-1'), isFalse);
+
+    await tester.tap(heart);
+    await tester.pumpAndSettle();
+    expect(await saved.isSaved('${'ab' * 32}:lot-1'), isTrue);
+
+    await tester.tap(heart);
+    await tester.pumpAndSettle();
+    expect(await saved.isSaved('${'ab' * 32}:lot-1'), isFalse);
+  });
+
+  testWidgets('no heart on my own listing', (tester) async {
+    final social = await SocialService.fromRootSeedHex('00' * 32);
+    final settings = SocialSettings(InMemorySecretStore());
+    await settings.setRelayUrls(const []);
+    final connection = SocialConnection(social: social, settings: settings);
+    final saved = SavedOffersStore(InMemorySecretStore());
+
+    await tester.pumpWidget(_wrap(MarketOfferDetailScreen(
+      offer: _offer(),
+      connection: connection,
+      mine: true,
+      savedOffers: saved,
+    )));
+    await tester.pumpAndSettle();
+
+    expect(find.byKey(const Key('offerDetail.save')), findsNothing);
+  });
 }