feat(messages): unread badges + OS notification for private messages

Private messages (NIP-17) arrived in the foreground with no signal: no
unread count, no badge, no OS notification. Hook both into the single
InboxService.ingest choke point.

- UnreadService: per-peer "last read" in the keystore (timestamps+pubkey
  only, no message text), live count via a changes stream, active-peer
  suppression so the open chat never badges or notifies.
- UnreadBadge: reusable, RTL-aware (Flutter Badge) — on the home hamburger,
  the drawer "Chat" item, and per-conversation in the messages list.
- Mark-read on chat open (ChatScreen).
- NotificationService over flutter_local_notifications: generic
  "New message from <name>" (no message text, for privacy), payload = pubkey,
  tap opens /chat/:pubkey. No-op on web/Windows; Android POST_NOTIFICATIONS.
- i18n notifications.newMessageFrom (en/es/pt/ast).

Background/push stays out of scope (foreground-only by design).

Tests: UnreadService, NotificationService (mock plugin), InboxService hooks
(new peer notifies/counts; duplicate, own, and open-chat do not), and a
UnreadBadge widget test. dart analyze clean; commons_core green.

apps/app_seeds/test/services/inbox_service_test.dart

@@ -2,11 +2,30 @@ import 'package:commons_core/commons_core.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:tane/services/inbox_service.dart';
 import 'package:tane/services/message_store.dart';
+import 'package:tane/services/notification_service.dart';
 import 'package:tane/services/social_service.dart';
 import 'package:tane/services/social_settings.dart';
+import 'package:tane/services/unread_service.dart';
 
 import '../support/test_support.dart';
 
+/// Records notification calls instead of touching the OS plugin.
+class _RecordingNotifications extends NotificationService {
+  _RecordingNotifications() : super(supported: false);
+
+  final titles = <String>[];
+  final peers = <String>[];
+
+  @override
+  Future<void> showMessage({
+    required String peerPubkey,
+    required String title,
+  }) async {
+    peers.add(peerPubkey);
+    titles.add(title);
+  }
+}
+
 /// The app-wide inbox listener persists incoming messages and announces changes,
 /// so the inbox list refreshes even when the specific chat isn't open. Driven
 /// through the [InboxService.ingest] seam so no relay/network is involved.
@@ -52,4 +71,52 @@ void main() {
     expect(await store.history('alice'), hasLength(1));
     await sub.cancel();
   });
+
+  group('unread + notification hooks', () {
+    late UnreadService unread;
+    late _RecordingNotifications notifications;
+    late String myPubkey;
+
+    setUp(() async {
+      final social = await SocialService.fromRootSeedHex(seedHex);
+      myPubkey = social.publicKeyHex;
+      unread = UnreadService(store, InMemorySecretStore());
+      notifications = _RecordingNotifications();
+      inbox = InboxService(
+        social: social,
+        settings: SocialSettings(InMemorySecretStore()),
+        store: store,
+        unread: unread,
+        notifications: notifications,
+      );
+    });
+
+    tearDown(() => unread.dispose());
+
+    test('a new peer message updates unread and notifies', () async {
+      await inbox.ingest(msg('alice', 'hola', 1000));
+      expect(await unread.unreadCount('alice'), 1);
+      expect(notifications.peers, ['alice']);
+    });
+
+    test('a duplicate re-delivery neither counts nor notifies', () async {
+      await inbox.ingest(msg('alice', 'hola', 1000));
+      await inbox.ingest(msg('alice', 'hola', 1000));
+      expect(await unread.unreadCount('alice'), 1);
+      expect(notifications.peers, hasLength(1));
+    });
+
+    test('a message authored by me is never notified', () async {
+      // NIP-17 doesn't loop your own gift wrap back; the guard is defensive.
+      await inbox.ingest(msg(myPubkey, 'echo', 1000));
+      expect(notifications.peers, isEmpty);
+    });
+
+    test('the open chat is not notified and stays read', () async {
+      unread.activePeer = 'alice';
+      await inbox.ingest(msg('alice', 'while open', 1000));
+      expect(await unread.unreadCount('alice'), 0);
+      expect(notifications.peers, isEmpty);
+    });
+  });
 }