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/notification_service_test.dart

@@ -0,0 +1,78 @@
+import 'package:flutter_local_notifications/flutter_local_notifications.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:mocktail/mocktail.dart';
+import 'package:tane/services/notification_service.dart';
+
+class _MockPlugin extends Mock implements FlutterLocalNotificationsPlugin {}
+
+void main() {
+  late _MockPlugin plugin;
+
+  setUpAll(() {
+    registerFallbackValue(
+      const NotificationDetails(
+        android: AndroidNotificationDetails('x', 'x'),
+      ),
+    );
+    registerFallbackValue(const InitializationSettings());
+  });
+
+  setUp(() {
+    plugin = _MockPlugin();
+    when(() => plugin.show(any(), any(), any(), any(),
+        payload: any(named: 'payload'))).thenAnswer((_) async {});
+    // Generic platform resolution returns null, so the permission requests in
+    // initialize() are skipped in tests.
+    when(() => plugin.resolvePlatformSpecificImplementation())
+        .thenReturn(null);
+  });
+
+  test('showMessage passes the title and pubkey payload, no body', () async {
+    final service = NotificationService(plugin: plugin, supported: true);
+    await service.showMessage(
+        peerPubkey: 'abc123', title: 'New message from Alice');
+    final captured = verify(() => plugin.show(
+          captureAny(),
+          captureAny(),
+          captureAny(),
+          captureAny(),
+          payload: captureAny(named: 'payload'),
+        )).captured;
+    expect(captured[1], 'New message from Alice'); // title
+    expect(captured[2], isNull); // no body (privacy: no message text)
+    expect(captured[4], 'abc123'); // payload = peer pubkey
+  });
+
+  test('is a no-op on unsupported platforms (web/windows)', () async {
+    final service = NotificationService(plugin: plugin, supported: false);
+    await service.initialize();
+    await service.showMessage(peerPubkey: 'abc123', title: 'hi');
+    verifyNever(() => plugin.show(any(), any(), any(), any(),
+        payload: any(named: 'payload')));
+  });
+
+  test('a tap with a payload invokes onTapChat with the pubkey', () async {
+    // Capture the response handler initialize() registers, then simulate a tap.
+    void Function(NotificationResponse)? handler;
+    when(() => plugin.initialize(
+      any(),
+      onDidReceiveNotificationResponse:
+          any(named: 'onDidReceiveNotificationResponse'),
+    )).thenAnswer((inv) async {
+      handler = inv.namedArguments[#onDidReceiveNotificationResponse]
+          as void Function(NotificationResponse)?;
+      return true;
+    });
+
+    final service = NotificationService(plugin: plugin, supported: true);
+    String? tapped;
+    service.onTapChat = (pk) => tapped = pk;
+    await service.initialize();
+
+    handler!(const NotificationResponse(
+      notificationResponseType: NotificationResponseType.selectedNotification,
+      payload: 'abc123',
+    ));
+    expect(tapped, 'abc123');
+  });
+}