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/di/injector.dart

@@ -23,6 +23,7 @@ import '../services/file_picker_file_service.dart';
 import '../services/file_service.dart';
 import '../services/inbox_service.dart';
 import '../services/locale_store.dart';
+import '../services/notification_service.dart';
 import '../services/ocr/label_text_extractor.dart';
 import '../services/ocr/ocr_language.dart';
 import '../services/ocr/tesseract_label_extractor.dart';
@@ -36,6 +37,7 @@ import '../services/profile_cache.dart';
 import '../services/profile_store.dart';
 import '../services/social_service.dart';
 import '../services/social_settings.dart';
+import '../services/unread_service.dart';
 
 /// The app's service locator. Kept to the composition root — widgets get their
 /// repositories from here (or via BlocProvider), never by reaching into it deep
@@ -169,19 +171,32 @@ Future<void> configureDependencies() async {
     );
   }
 
+  // OS notifications for foreground messages. Registered unconditionally — it
+  // self-no-ops on unsupported platforms (web/Windows) and when the social layer
+  // is absent nothing ever calls it. Its tap handler is wired to the router in
+  // `TaneApp`, after the router exists.
+  getIt.registerSingleton<NotificationService>(NotificationService());
+
   // Optional: absent only if the derivation above failed — then the app runs
   // inventory-only, by design (market/chat/profile hidden).
   if (socialService != null) {
     getIt
       ..registerSingleton<SocialService>(socialService)
+      // Tracks unread private messages so the UI can badge them.
+      ..registerSingleton<UnreadService>(
+        UnreadService(getIt<MessageStore>(), secretStore),
+      )
       // App-wide inbox listener so private messages arrive (and land in the
       // inbox list) even when the specific chat isn't open. Started in `main`.
+      // It also updates unread counts and fires notifications.
       ..registerSingleton<InboxService>(
         InboxService(
           social: socialService,
           settings: getIt<SocialSettings>(),
           store: getIt<MessageStore>(),
           profileCache: getIt<ProfileCache>(),
+          unread: getIt<UnreadService>(),
+          notifications: getIt<NotificationService>(),
         ),
       );
   }