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/lib/services/saved_offers_store.dart

@@ -0,0 +1,123 @@
+import 'dart:async';
+import 'dart:convert';
+
+import 'package:commons_core/commons_core.dart';
+
+import '../security/secret_store.dart';
+
+/// A person's saved ("favorite") market offers — other people's listings they
+/// bookmarked, Wallapop-style. Keystore-backed (no plaintext at rest) and
+/// namespaced by the active identity's [accountScope], like [ProfileStore].
+///
+/// We keep a *snapshot* of each offer (not just its coordinate) so the Favorites
+/// list renders offline, and so a since-withdrawn offer can still be shown as
+/// "no longer available" instead of vanishing. The stable key is the offer's
+/// addressable coordinate `authorPubkeyHex:id` ([keyOf]).
+class SavedOffersStore {
+  SavedOffersStore(this._store, {String accountScope = ''})
+    : _key = accountScope.isEmpty
+          ? 'tane.social.saved_offers'
+          : 'tane.social.$accountScope.saved_offers';
+
+  final SecretStore _store;
+  final String _key;
+
+  final _changes = StreamController<void>.broadcast();
+
+  /// Emits whenever the saved set changes, so open screens can refresh.
+  Stream<void> get changes => _changes.stream;
+
+  /// The stable per-offer key: its NIP-99 addressable coordinate without the
+  /// kind (`authorPubkeyHex:id`). Same offer → same key on every device.
+  static String keyOf(Offer offer) => '${offer.authorPubkeyHex}:${offer.id}';
+
+  /// Every saved offer, newest first (by save time).
+  Future<List<Offer>> list() async {
+    final raw = await _store.read(_key);
+    if (raw == null || raw.isEmpty) return const [];
+    final entries = (jsonDecode(raw) as List)
+        .cast<Map<String, dynamic>>()
+        .toList()
+      ..sort((a, b) => (b['savedAt'] as int).compareTo(a['savedAt'] as int));
+    return [for (final e in entries) _decode(e)];
+  }
+
+  /// Whether [key] (from [keyOf]) is currently saved.
+  Future<bool> isSaved(String key) async =>
+      (await _rawList()).any((e) => e['key'] == key);
+
+  /// Saves [offer] (idempotent on its key). Newer snapshot replaces an older one.
+  Future<void> save(Offer offer, {required int savedAt}) async {
+    final key = keyOf(offer);
+    final list = await _rawList()..removeWhere((e) => e['key'] == key);
+    list.add({..._encode(offer), 'key': key, 'savedAt': savedAt});
+    await _writeRaw(list);
+  }
+
+  /// Removes the saved offer with [key] (from [keyOf]); no-op if absent.
+  Future<void> remove(String key) async {
+    final list = await _rawList()..removeWhere((e) => e['key'] == key);
+    await _writeRaw(list);
+  }
+
+  /// Toggles [offer]'s saved state; returns the new state (true = now saved).
+  Future<bool> toggle(Offer offer, {required int savedAt}) async {
+    if (await isSaved(keyOf(offer))) {
+      await remove(keyOf(offer));
+      return false;
+    }
+    await save(offer, savedAt: savedAt);
+    return true;
+  }
+
+  Future<List<Map<String, dynamic>>> _rawList() async {
+    final raw = await _store.read(_key);
+    if (raw == null || raw.isEmpty) return [];
+    return (jsonDecode(raw) as List).cast<Map<String, dynamic>>().toList();
+  }
+
+  Future<void> _writeRaw(List<Map<String, dynamic>> list) async {
+    await _store.write(_key, jsonEncode(list));
+    if (!_changes.isClosed) _changes.add(null);
+  }
+
+  Map<String, dynamic> _encode(Offer o) => {
+    'id': o.id,
+    'author': o.authorPubkeyHex,
+    'summary': o.summary,
+    'type': o.type.name,
+    'status': o.status.name,
+    if (o.category != null) 'category': o.category,
+    'geohash': o.approxGeohash,
+    if (o.radiusKm != null) 'radiusKm': o.radiusKm,
+    if (o.priceAmount != null) 'priceAmount': o.priceAmount,
+    if (o.priceCurrency != null) 'priceCurrency': o.priceCurrency,
+    if (o.exchangeTerms != null) 'exchangeTerms': o.exchangeTerms,
+    if (o.imageUrl != null) 'imageUrl': o.imageUrl,
+    if (o.expiresAt != null) 'expiresAt': o.expiresAt!.millisecondsSinceEpoch,
+    'isOrganic': o.isOrganic,
+  };
+
+  Offer _decode(Map<String, dynamic> e) => Offer(
+    id: e['id'] as String,
+    authorPubkeyHex: e['author'] as String,
+    summary: e['summary'] as String,
+    type: OfferType.values.byName(e['type'] as String),
+    status: OfferLifecycle.values.byName(e['status'] as String? ?? 'active'),
+    category: e['category'] as String?,
+    approxGeohash: e['geohash'] as String? ?? '',
+    radiusKm: e['radiusKm'] as int?,
+    priceAmount: e['priceAmount'] as num?,
+    priceCurrency: e['priceCurrency'] as String?,
+    exchangeTerms: e['exchangeTerms'] as String?,
+    imageUrl: e['imageUrl'] as String?,
+    expiresAt: e['expiresAt'] == null
+        ? null
+        : DateTime.fromMillisecondsSinceEpoch(e['expiresAt'] as int),
+    isOrganic: e['isOrganic'] as bool? ?? false,
+  );
+
+  Future<void> close() async {
+    await _changes.close();
+  }
+}