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);
+  });
+}