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/services/inbox_service.dart

@@ -4,10 +4,13 @@ import 'package:commons_core/commons_core.dart';
 import 'package:connectivity_plus/connectivity_plus.dart';
 import 'package:flutter/foundation.dart';
 
+import '../i18n/strings.g.dart';
 import 'message_store.dart';
+import 'notification_service.dart';
 import 'profile_cache.dart';
 import 'social_service.dart';
 import 'social_settings.dart';
+import 'unread_service.dart';
 
 /// App-wide inbox listener for private messages (NIP-17).
 ///
@@ -26,15 +29,21 @@ class InboxService {
     required SocialSettings settings,
     required MessageStore store,
     ProfileCache? profileCache,
+    UnreadService? unread,
+    NotificationService? notifications,
   })  : _social = social,
         _settings = settings,
         _store = store,
-        _profileCache = profileCache;
+        _profileCache = profileCache,
+        _unread = unread,
+        _notifications = notifications;
 
   final SocialService _social;
   final SocialSettings _settings;
   final MessageStore _store;
   final ProfileCache? _profileCache;
+  final UnreadService? _unread;
+  final NotificationService? _notifications;
 
   final _changes = StreamController<void>.broadcast();
   SocialSession? _session;
@@ -101,7 +110,29 @@ class InboxService {
     final stored = await _store.append(message.fromPubkey, message);
     if (!stored) return; // a re-delivered duplicate
     if (!_changes.isClosed) _changes.add(null);
-    await _cacheName(message.fromPubkey);
+
+    final peer = message.fromPubkey;
+    // Defensive: never badge or notify for your own message. NIP-17 doesn't
+    // loop a sent gift wrap back to its author, so this shouldn't happen — the
+    // guard just keeps the seam honest.
+    if (peer != _social.publicKeyHex) {
+      await _unread?.onMessageReceived(peer, message);
+      await _maybeNotify(peer);
+    }
+    await _cacheName(peer);
+  }
+
+  /// Fires a text-free OS notification for a new message from [peer], unless its
+  /// chat is the one already on screen (then the message is visible anyway).
+  Future<void> _maybeNotify(String peer) async {
+    final notifications = _notifications;
+    if (notifications == null) return;
+    if (_unread?.activePeer == peer) return;
+    final name = await _profileCache?.name(peer) ?? shortPubkey(peer);
+    await notifications.showMessage(
+      peerPubkey: peer,
+      title: t.notifications.newMessageFrom(name: name),
+    );
   }
 
   Future<void> _cacheName(String peerPubkey) async {