feat(block2): offline outbox — share now, publish when connected

Sender-side offline delivery hardening.
- OfferOutbox: a keystore-backed (no plaintext) queue of lot ids to publish
  later. Stores ids, not offers, so on flush each offer is rebuilt from the
  lot's CURRENT state — an offline edit is respected, a deleted lot drops out.
- flushOutbox(): on reconnect, republishes queued lots and clears them (no-op
  offline; drops ids whose lot is gone).
- Market screen: sharing offline parks the lots ('we'll share these when you're
  connected'); opening the market online auto-flushes the queue first.
- Wired via DI + TaneApp(outbox); i18n en/es/pt.

Tests: 4 outbox + 3 flush + market UI, 20 green. Analyzer clean for new code.

apps/app_seeds/test/services/offer_outbox_test.dart

@@ -0,0 +1,31 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:tane/services/offer_outbox.dart';
+
+import '../support/test_support.dart';
+
+void main() {
+  late OfferOutbox outbox;
+  setUp(() => outbox = OfferOutbox(InMemorySecretStore()));
+
+  test('starts empty', () async {
+    expect(await outbox.pending(), isEmpty);
+  });
+
+  test('enqueue adds ids; it is a set (no duplicates)', () async {
+    await outbox.enqueue(['a', 'b']);
+    await outbox.enqueue(['b', 'c']);
+    expect(await outbox.pending(), {'a', 'b', 'c'});
+  });
+
+  test('remove drops the given ids', () async {
+    await outbox.enqueue(['a', 'b', 'c']);
+    await outbox.remove(['b']);
+    expect(await outbox.pending(), {'a', 'c'});
+  });
+
+  test('clear empties it', () async {
+    await outbox.enqueue(['a', 'b']);
+    await outbox.clear();
+    expect(await outbox.pending(), isEmpty);
+  });
+}

apps/app_seeds/test/state/offers_cubit_test.dart

@@ -5,8 +5,11 @@ import 'package:flutter_test/flutter_test.dart';
 import 'package:tane/data/variety_repository.dart';
 import 'package:tane/db/enums.dart';
 import 'package:tane/services/offer_mapper.dart';
+import 'package:tane/services/offer_outbox.dart';
 import 'package:tane/state/offers_cubit.dart';
 
+import '../support/test_support.dart';
+
 /// In-memory [OfferTransport] for tests: addressable by id, discover matches by
 /// geohash prefix. No relay, no network.
 class FakeOfferTransport implements OfferTransport {
@@ -196,6 +199,63 @@ void main() {
       await noArea.close();
     });
 
+    test('flushOutbox publishes queued lots, then clears them', () async {
+      final outbox = OfferOutbox(InMemorySecretStore());
+      await outbox.enqueue(['lot-1', 'lot-2']);
+      final cubit = OffersCubit(FakeOfferTransport());
+      final count = await flushOutbox(
+        outbox: outbox,
+        cubit: cubit,
+        shareableLots: const [
+          ShareableLot(
+              lotId: 'lot-1', summary: 'Tomate', offerStatus: OfferStatus.shared),
+          ShareableLot(
+              lotId: 'lot-2',
+              summary: 'Judía',
+              offerStatus: OfferStatus.exchange),
+          ShareableLot(
+              lotId: 'lot-3', summary: 'Otro', offerStatus: OfferStatus.shared),
+        ],
+        authorPubkeyHex: 'ab' * 32,
+        areaGeohash: 'sp3e9',
+      );
+      expect(count, 2);
+      expect(await outbox.pending(), isEmpty);
+      await cubit.close();
+    });
+
+    test('flushOutbox offline keeps the queue for later', () async {
+      final outbox = OfferOutbox(InMemorySecretStore());
+      await outbox.enqueue(['lot-1']);
+      final cubit = OffersCubit(null); // offline
+      final count = await flushOutbox(
+        outbox: outbox,
+        cubit: cubit,
+        shareableLots: const [],
+        authorPubkeyHex: 'ab' * 32,
+        areaGeohash: 'sp3e9',
+      );
+      expect(count, 0);
+      expect(await outbox.pending(), {'lot-1'});
+      await cubit.close();
+    });
+
+    test('flushOutbox drops queued lots that no longer exist', () async {
+      final outbox = OfferOutbox(InMemorySecretStore());
+      await outbox.enqueue(['gone']);
+      final cubit = OffersCubit(FakeOfferTransport());
+      final count = await flushOutbox(
+        outbox: outbox,
+        cubit: cubit,
+        shareableLots: const [],
+        authorPubkeyHex: 'ab' * 32,
+        areaGeohash: 'sp3e9',
+      );
+      expect(count, 0);
+      expect(await outbox.pending(), isEmpty);
+      await cubit.close();
+    });
+
     test('offline (no transport): degrades gracefully, never throws', () async {
       final cubit = OffersCubit(null);
       expect(cubit.isOnline, isFalse);