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/lib/di/injector.dart

@@ -24,6 +24,7 @@ import '../services/ocr/tesseract_label_extractor.dart';
 import '../services/onboarding_store.dart';
 import '../services/recovery_sheet_service.dart';
 import '../services/share_catalog_service.dart';
+import '../services/offer_outbox.dart';
 import '../services/social_service.dart';
 import '../services/social_settings.dart';
 
@@ -85,6 +86,7 @@ Future<void> configureDependencies() async {
     ..registerSingleton<OnboardingStore>(OnboardingStore(secretStore))
     ..registerSingleton<SocialService>(socialService)
     ..registerSingleton<SocialSettings>(SocialSettings(secretStore))
+    ..registerSingleton<OfferOutbox>(OfferOutbox(secretStore))
     ..registerSingleton<ExportImportService>(
       ExportImportService(
         repository: varietyRepository,