feat(block2): messages inbox + drawer 'Chat' link (was unreachable)

Chat was only reachable via an offer's 'Message' button, and the drawer 'Chat'
sat on 'coming soon' — so messaging/trust were effectively hidden.
- MessageStore keeps a conversation index (the keystore has no key enumeration)
  and exposes conversations() — peers newest-active-first with their last line.
- ChatListScreen: the messages inbox (tap a conversation → /chat/:pubkey).
- Drawer 'Chat' is now a live destination -> /messages (gated like Market).
- i18n en/es/pt. conversations() covered by a plain test (no widget hang).

Analyzer clean.

apps/app_seeds/lib/services/message_store.dart

@@ -8,12 +8,29 @@ import '../security/secret_store.dart';
 /// 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.
+/// A conversation preview for the messages inbox.
+class ChatSummary {
+  const ChatSummary({
+    required this.peerPubkey,
+    required this.lastText,
+    required this.lastAt,
+  });
+
+  final String peerPubkey;
+  final String lastText;
+  final DateTime lastAt;
+}
+
 class MessageStore {
   MessageStore(this._store);
 
   final SecretStore _store;
   static const _prefix = 'tane.social.chat.';
 
+  /// Index of peers we have a conversation with (the keystore is key/value with
+  /// no key enumeration, so we track the list ourselves).
+  static const _indexKey = 'tane.social.chats';
+
   /// Keep only the most recent [_cap] messages per conversation, to bound the
   /// keystore entry size.
   static const _cap = 200;
@@ -36,7 +53,7 @@ class MessageStore {
   }
 
   /// Appends [message] to the conversation with [peerPubkey] (trimming to the
-  /// cap).
+  /// cap) and records the peer in the conversation index.
   Future<void> append(String peerPubkey, PrivateMessage message) async {
     final next = [...await history(peerPubkey), message];
     final capped =
@@ -52,5 +69,35 @@ class MessageStore {
           },
       ]),
     );
+    await _rememberPeer(peerPubkey);
+  }
+
+  /// Conversations, most-recently-active first (for the messages inbox).
+  Future<List<ChatSummary>> conversations() async {
+    final summaries = <ChatSummary>[];
+    for (final peer in await _peers()) {
+      final history = await this.history(peer);
+      if (history.isEmpty) continue;
+      final last = history.last;
+      summaries.add(ChatSummary(
+        peerPubkey: peer,
+        lastText: last.text,
+        lastAt: last.at,
+      ));
+    }
+    summaries.sort((a, b) => b.lastAt.compareTo(a.lastAt));
+    return summaries;
+  }
+
+  Future<List<String>> _peers() async {
+    final raw = await _store.read(_indexKey);
+    if (raw == null || raw.isEmpty) return const [];
+    return raw.split('\n').where((s) => s.isNotEmpty).toList();
+  }
+
+  Future<void> _rememberPeer(String peer) async {
+    final peers = await _peers();
+    if (peers.contains(peer)) return;
+    await _store.write(_indexKey, [...peers, peer].join('\n'));
   }
 }