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/main.dart

@@ -7,6 +7,7 @@ import 'di/injector.dart';
 import 'i18n/strings.g.dart';
 import 'services/auto_backup_service.dart';
 import 'services/coarse_location.dart';
+import 'services/message_store.dart';
 import 'services/offer_outbox.dart';
 import 'services/onboarding_store.dart';
 import 'services/social_service.dart';
@@ -27,6 +28,7 @@ Future<void> main() async {
         socialSettings: getIt<SocialSettings>(),
         location: const GeolocatorCoarseLocation(),
         outbox: getIt<OfferOutbox>(),
+        messageStore: getIt<MessageStore>(),
         showIntro: !await onboarding.introSeen(),
         autoBackup: getIt.isRegistered<AutoBackupService>()
             ? getIt<AutoBackupService>()