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(

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,

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>()

apps/app_seeds/lib/services/message_store.dart

@@ -0,0 +1,56 @@
+import 'dart:convert';
+
+import 'package:commons_core/commons_core.dart';
+
+import '../security/secret_store.dart';
+
+/// Persists a 1:1 chat history, keystore-backed (so no plaintext at rest). A
+/// capped, per-peer JSON list — recent history survives leaving the chat. Small
+/// and simple on purpose; the encrypted Drift DB is the eventual home if chats
+/// grow large.
+class MessageStore {
+  MessageStore(this._store);
+
+  final SecretStore _store;
+  static const _prefix = 'tane.social.chat.';
+
+  /// Keep only the most recent [_cap] messages per conversation, to bound the
+  /// keystore entry size.
+  static const _cap = 200;
+
+  String _key(String peerPubkey) => '$_prefix$peerPubkey';
+
+  /// Messages exchanged with [peerPubkey], oldest first.
+  Future<List<PrivateMessage>> history(String peerPubkey) async {
+    final raw = await _store.read(_key(peerPubkey));
+    if (raw == null || raw.isEmpty) return const [];
+    final list = jsonDecode(raw) as List;
+    return [
+      for (final m in list.cast<Map<String, dynamic>>())
+        PrivateMessage(
+          fromPubkey: m['from'] as String,
+          text: m['text'] as String,
+          at: DateTime.fromMillisecondsSinceEpoch(m['at'] as int),
+        ),
+    ];
+  }
+
+  /// Appends [message] to the conversation with [peerPubkey] (trimming to the
+  /// cap).
+  Future<void> append(String peerPubkey, PrivateMessage message) async {
+    final next = [...await history(peerPubkey), message];
+    final capped =
+        next.length > _cap ? next.sublist(next.length - _cap) : next;
+    await _store.write(
+      _key(peerPubkey),
+      jsonEncode([
+        for (final m in capped)
+          {
+            'from': m.fromPubkey,
+            'text': m.text,
+            'at': m.at.millisecondsSinceEpoch,
+          },
+      ]),
+    );
+  }
+}

apps/app_seeds/lib/state/messages_cubit.dart

@@ -4,6 +4,7 @@ import 'package:commons_core/commons_core.dart';
 import 'package:equatable/equatable.dart';
 import 'package:flutter_bloc/flutter_bloc.dart';
 
+import '../services/message_store.dart';
 import '../services/social_service.dart';
 import '../services/social_settings.dart';
 
@@ -46,11 +47,14 @@ class MessagesCubit extends Cubit<ChatState> {
     this._transport, {
     required this.peerPubkey,
     required this.selfPubkey,
+    MessageStore? store,
     Future<void> Function()? onDispose,
-  })  : _onDispose = onDispose,
+  })  : _store = store,
+        _onDispose = onDispose,
         super(const ChatState());
 
   final MessageTransport? _transport;
+  final MessageStore? _store;
   final String peerPubkey;
   final String selfPubkey;
   final Future<void> Function()? _onDispose;
@@ -58,17 +62,26 @@ class MessagesCubit extends Cubit<ChatState> {
 
   bool get isOnline => _transport != null;
 
-  /// Subscribes to incoming messages from [peerPubkey].
-  void start() {
+  /// Subscribes to incoming messages, then loads any saved history. Subscribing
+  /// first (before the async history load) avoids dropping an event that arrives
+  /// during the load.
+  Future<void> start() async {
     final transport = _transport;
-    if (transport == null) return;
-    _sub = transport.inbox().listen(
-      (message) {
-        if (message.fromPubkey != peerPubkey) return; // another conversation
-        emit(state.copyWith(messages: [...state.messages, message]));
-      },
-      onError: (Object e) => emit(state.copyWith(error: () => '$e')),
-    );
+    if (transport != null) {
+      _sub = transport.inbox().listen(
+        (message) async {
+          if (message.fromPubkey != peerPubkey) return; // another conversation
+          await _store?.append(peerPubkey, message);
+          emit(state.copyWith(messages: [...state.messages, message]));
+        },
+        onError: (Object e) => emit(state.copyWith(error: () => '$e')),
+      );
+    }
+    // History is older than anything that arrives now, so prepend it.
+    final history = await _store?.history(peerPubkey);
+    if (history != null && history.isNotEmpty) {
+      emit(state.copyWith(messages: [...history, ...state.messages]));
+    }
   }
 
   /// Sends [text] to the peer and appends it optimistically.
@@ -79,16 +92,15 @@ class MessagesCubit extends Cubit<ChatState> {
     emit(state.copyWith(sending: true, error: () => null));
     try {
       await transport.send(toPubkey: peerPubkey, text: trimmed);
+      final mine = PrivateMessage(
+        fromPubkey: selfPubkey,
+        text: trimmed,
+        at: DateTime.now(),
+      );
+      await _store?.append(peerPubkey, mine);
       emit(state.copyWith(
         sending: false,
-        messages: [
-          ...state.messages,
-          PrivateMessage(
-            fromPubkey: selfPubkey,
-            text: trimmed,
-            at: DateTime.now(),
-          ),
-        ],
+        messages: [...state.messages, mine],
       ));
     } catch (e) {
       emit(state.copyWith(sending: false, error: () => '$e'));

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());