feat(block2): persist chat history (messages survive leaving the chat)

Messages were in-memory only — gone on leaving the conversation.
- MessageStore: keystore-backed (no plaintext), a capped per-peer JSON list
  (last 200). Simple; the encrypted Drift DB is the eventual home at scale.
- MessagesCubit: loads saved history on start (subscribes FIRST, then loads, so
  a message arriving during the load isn't dropped) and persists every sent and
  received message. Wired via DI + TaneApp(messageStore).

Tests (plain 'test', no hang risk): MessageStore round-trip/per-peer/cap, and a
cubit test that reopens a fresh cubit and sees the saved conversation. Analyzer
clean; run 'flutter test' locally to confirm.

apps/app_seeds/lib/di/injector.dart

@@ -27,6 +27,7 @@ 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/message_store.dart';
 import '../services/offer_outbox.dart';
 import '../services/social_service.dart';
 import '../services/social_settings.dart';
@@ -90,6 +91,7 @@ Future<void> configureDependencies() async {
     ..registerSingleton<SocialService>(socialService)
     ..registerSingleton<SocialSettings>(SocialSettings(secretStore))
     ..registerSingleton<OfferOutbox>(OfferOutbox(secretStore))
+    ..registerSingleton<MessageStore>(MessageStore(secretStore))
     ..registerSingleton<ExportImportService>(
       ExportImportService(
         repository: varietyRepository,