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/lib/app.dart

@@ -17,6 +17,7 @@ import 'state/inventory_cubit.dart';
 import 'state/variety_detail_cubit.dart';
 import 'ui/about_screen.dart';
 import 'ui/auto_backup_gate.dart';
+import 'ui/chat_screen.dart';
 import 'ui/home_screen.dart';
 import 'ui/intro_screen.dart';
 import 'ui/inventory_list_screen.dart';
@@ -98,6 +99,15 @@ class TaneApp extends StatelessWidget {
               outbox: outbox,
             ),
           ),
+        if (social != null && socialSettings != null)
+          GoRoute(
+            path: '/chat/:pubkey',
+            builder: (context, state) => ChatScreen(
+              social: social,
+              settings: socialSettings,
+              peerPubkey: state.pathParameters['pubkey']!,
+            ),
+          ),
         GoRoute(
           path: '/intro',
           builder: (context, state) => IntroScreen(