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/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,
+          },
+      ]),
+    );
+  }
+}