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/lib/bootstrap.dart

@@ -13,6 +13,7 @@ import 'services/coarse_location.dart';
 import 'services/inbox_service.dart';
 import 'services/locale_store.dart';
 import 'services/message_store.dart';
+import 'services/notification_service.dart';
 import 'services/offer_outbox.dart';
 import 'services/onboarding_store.dart';
 import 'services/profile_cache.dart';
@@ -51,6 +52,12 @@ class _BootstrapState extends State<Bootstrap> {
     final onboarding = getIt<OnboardingStore>();
     final inbox =
         getIt.isRegistered<InboxService>() ? getIt<InboxService>() : null;
+    final notifications = getIt.isRegistered<NotificationService>()
+        ? getIt<NotificationService>()
+        : null;
+    // Ask for notification permission and set up the OS channel (no-op on
+    // unsupported platforms). Done before the inbox starts listening.
+    if (notifications != null) await notifications.initialize();
     // Listen for incoming private messages app-wide (foreground) so they arrive
     // and fill the inbox even before their chat is opened.
     if (inbox != null) unawaited(inbox.start());
@@ -68,6 +75,7 @@ class _BootstrapState extends State<Bootstrap> {
       profileStore: getIt<ProfileStore>(),
       profileCache: getIt<ProfileCache>(),
       inbox: inbox,
+      notifications: notifications,
       showIntro: !await onboarding.introSeen(),
       autoBackup: getIt.isRegistered<AutoBackupService>()
           ? getIt<AutoBackupService>()