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/unread_service_test.dart

@@ -0,0 +1,90 @@
+import 'package:commons_core/commons_core.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:tane/services/message_store.dart';
+import 'package:tane/services/unread_service.dart';
+
+import '../support/test_support.dart';
+
+/// Unread counting is derived from stored history plus a per-peer "last read"
+/// mark; these tests drive it through a real [MessageStore] over an in-memory
+/// keystore, so no relay or network is involved.
+void main() {
+  const me = 'me-pubkey';
+
+  late MessageStore store;
+  late UnreadService unread;
+
+  setUp(() {
+    store = MessageStore(InMemorySecretStore());
+    unread = UnreadService(store, InMemorySecretStore());
+  });
+
+  tearDown(() => unread.dispose());
+
+  PrivateMessage msg(String from, String text, int atMs) => PrivateMessage(
+      fromPubkey: from,
+      text: text,
+      at: DateTime.fromMillisecondsSinceEpoch(atMs));
+
+  // Persist then notify, the way InboxService.ingest does.
+  Future<void> receive(String peer, PrivateMessage m) async {
+    await store.append(peer, m);
+    await unread.onMessageReceived(peer, m);
+  }
+
+  test('an unknown peer has no unread', () async {
+    expect(await unread.unreadCount('alice'), 0);
+  });
+
+  test('inbound messages increment the unread count', () async {
+    await receive('alice', msg('alice', 'hi', 1000));
+    await receive('alice', msg('alice', 'you there?', 2000));
+    expect(await unread.unreadCount('alice'), 2);
+  });
+
+  test('markRead clears older messages but keeps newer ones', () async {
+    await receive('alice', msg('alice', 'a', 1000));
+    await unread.markRead('alice'); // reads up to the latest (1000)
+    expect(await unread.unreadCount('alice'), 0);
+
+    await receive('alice', msg('alice', 'b', 2000));
+    expect(await unread.unreadCount('alice'), 1);
+  });
+
+  test('own outgoing messages never count as unread', () async {
+    await receive('alice', msg(me, 'my reply', 1000));
+    expect(await unread.unreadCount('alice'), 0);
+  });
+
+  test('a message for the open chat is auto-read (no unread)', () async {
+    unread.activePeer = 'alice';
+    await receive('alice', msg('alice', 'while open', 1000));
+    expect(await unread.unreadCount('alice'), 0);
+  });
+
+  test('totalUnreadCount sums across conversations', () async {
+    await receive('alice', msg('alice', 'a', 1000));
+    await receive('bob', msg('bob', 'b1', 1000));
+    await receive('bob', msg('bob', 'b2', 2000));
+    expect(await unread.totalUnreadCount(), 3);
+  });
+
+  test('changes fires on a new inbound message', () async {
+    final events = <void>[];
+    final sub = unread.changes.listen(events.add);
+    await receive('alice', msg('alice', 'hi', 1000));
+    await Future<void>.delayed(Duration.zero);
+    expect(events, hasLength(1));
+    await sub.cancel();
+  });
+
+  test('changes fires on markRead', () async {
+    await receive('alice', msg('alice', 'hi', 1000));
+    final events = <void>[];
+    final sub = unread.changes.listen(events.add);
+    await unread.markRead('alice');
+    await Future<void>.delayed(Duration.zero);
+    expect(events, hasLength(1));
+    await sub.cancel();
+  });
+}