feat(block2): messaging UI — private 1:1 chat (NIP-17)

- MessagesCubit: 1:1 conversation over MessageTransport. Filters the shared
  inbox to the peer; tags our own sends with our pubkey (NIP-17 wraps don't come
  back to the sender) so the UI can align bubbles; degrades gracefully offline.
- ChatScreen (mockup 10): message bubbles (mine right / peer left) + a composer;
  opens a session lazily, shows 'set up sharing' when there's no relay.
- Entry point: a 'Message' button on each offer card (hidden on your own
  listings) → /chat/:pubkey.
- Routing + i18n en/es/pt.

Tests: 5 cubit (send/receive/order/filter/offline) + chat offline + market, green.

apps/app_seeds/test/state/messages_cubit_test.dart

@@ -0,0 +1,90 @@
+import 'dart:async';
+
+import 'package:commons_core/commons_core.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:tane/state/messages_cubit.dart';
+
+/// In-memory [MessageTransport]: records sends, lets a test push inbox messages.
+class FakeMessageTransport implements MessageTransport {
+  final List<({String to, String text})> sent = [];
+  final StreamController<PrivateMessage> _inbox =
+      StreamController<PrivateMessage>.broadcast();
+
+  void receive(PrivateMessage message) => _inbox.add(message);
+
+  @override
+  Future<void> send({required String toPubkey, required String text}) async =>
+      sent.add((to: toPubkey, text: text));
+
+  @override
+  Stream<PrivateMessage> inbox() => _inbox.stream;
+
+  @override
+  Future<void> close() async => _inbox.close();
+}
+
+void main() {
+  const peer = 'aa';
+  const me = 'bb';
+
+  PrivateMessage msg(String from, String text) =>
+      PrivateMessage(fromPubkey: from, text: text, at: DateTime(2026));
+
+  test('receives messages from the peer only', () async {
+    final transport = FakeMessageTransport();
+    final cubit =
+        MessagesCubit(transport, peerPubkey: peer, selfPubkey: me)..start();
+
+    transport.receive(msg(peer, 'hola'));
+    transport.receive(msg('cc', 'not for this chat')); // other conversation
+    await pumpEventQueue();
+
+    expect(cubit.state.messages, hasLength(1));
+    expect(cubit.state.messages.single.text, 'hola');
+    expect(cubit.isMine(cubit.state.messages.single), isFalse);
+    await cubit.close();
+  });
+
+  test('send delivers to the peer and appends our own message', () async {
+    final transport = FakeMessageTransport();
+    final cubit = MessagesCubit(transport, peerPubkey: peer, selfPubkey: me);
+
+    await cubit.send('  ¿cambiamos?  ');
+    expect(transport.sent, [(to: peer, text: '¿cambiamos?')]);
+    expect(cubit.state.messages, hasLength(1));
+    expect(cubit.isMine(cubit.state.messages.single), isTrue);
+    expect(cubit.state.sending, isFalse);
+    await cubit.close();
+  });
+
+  test('a full exchange keeps order and sides', () async {
+    final transport = FakeMessageTransport();
+    final cubit =
+        MessagesCubit(transport, peerPubkey: peer, selfPubkey: me)..start();
+
+    await cubit.send('hi');
+    transport.receive(msg(peer, 'hello'));
+    await pumpEventQueue();
+
+    expect(cubit.state.messages.map((m) => m.text), ['hi', 'hello']);
+    expect(cubit.state.messages.map(cubit.isMine), [true, false]);
+    await cubit.close();
+  });
+
+  test('empty/whitespace text is not sent', () async {
+    final transport = FakeMessageTransport();
+    final cubit = MessagesCubit(transport, peerPubkey: peer, selfPubkey: me);
+    await cubit.send('   ');
+    expect(transport.sent, isEmpty);
+    expect(cubit.state.messages, isEmpty);
+    await cubit.close();
+  });
+
+  test('offline (no transport) never throws', () async {
+    final cubit = MessagesCubit(null, peerPubkey: peer, selfPubkey: me)..start();
+    expect(cubit.isOnline, isFalse);
+    await cubit.send('hi');
+    expect(cubit.state.messages, isEmpty);
+    await cubit.close();
+  });
+}