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/ui/chat_screen.dart

@@ -4,6 +4,7 @@ import 'package:flutter/material.dart';
 import 'package:flutter_bloc/flutter_bloc.dart';
 
 import '../i18n/strings.g.dart';
+import '../services/message_store.dart';
 import '../services/social_service.dart';
 import '../services/social_settings.dart';
 import '../state/messages_cubit.dart';
@@ -18,6 +19,7 @@ class ChatScreen extends StatefulWidget {
     required this.social,
     required this.settings,
     required this.peerPubkey,
+    this.messageStore,
     super.key,
   });
 
@@ -25,6 +27,9 @@ class ChatScreen extends StatefulWidget {
   final SocialSettings settings;
   final String peerPubkey;
 
+  /// Optional persistence for chat history (keystore-backed); null in tests.
+  final MessageStore? messageStore;
+
   @override
   State<ChatScreen> createState() => _ChatScreenState();
 }
@@ -59,9 +64,12 @@ class _ChatScreenState extends State<ChatScreen> {
       return;
     }
     final self = widget.social.publicKeyHex;
-    final messages = MessagesCubit(session?.messages,
-        peerPubkey: widget.peerPubkey, selfPubkey: self)
-      ..start();
+    final messages = MessagesCubit(
+      session?.messages,
+      peerPubkey: widget.peerPubkey,
+      selfPubkey: self,
+      store: widget.messageStore,
+    )..start();
     final trust = TrustCubit(session?.trust,
         peerPubkey: widget.peerPubkey, selfPubkey: self);
     unawaited(trust.load());