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/app.dart

@@ -9,6 +9,7 @@ import 'data/variety_repository.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';
@@ -38,6 +39,7 @@ class TaneApp extends StatelessWidget {
     this.socialSettings,
     this.location,
     this.outbox,
+    this.messageStore,
     this.showIntro = false,
     this.autoBackup,
     super.key,
@@ -49,6 +51,7 @@ class TaneApp extends StatelessWidget {
           socialSettings,
           location,
           outbox,
+          messageStore,
         );
 
   final VarietyRepository repository;
@@ -65,6 +68,9 @@ class TaneApp extends StatelessWidget {
 
   /// Optional offline outbox for the market's "share my seeds".
   final OfferOutbox? outbox;
+
+  /// Optional persistence for chat history.
+  final MessageStore? messageStore;
   final bool showIntro;
 
   /// Drives silent periodic backups off the app lifecycle. Null in widget tests
@@ -80,6 +86,7 @@ class TaneApp extends StatelessWidget {
     SocialSettings? socialSettings,
     CoarseLocationProvider? location,
     OfferOutbox? outbox,
+    MessageStore? messageStore,
   ) {
     return GoRouter(
       initialLocation: showIntro ? '/intro' : '/',
@@ -106,6 +113,7 @@ class TaneApp extends StatelessWidget {
               social: social,
               settings: socialSettings,
               peerPubkey: state.pathParameters['pubkey']!,
+              messageStore: messageStore,
             ),
           ),
         GoRoute(