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/ui/chat_screen.dart

@@ -5,11 +5,13 @@ import 'package:flutter/services.dart';
 import 'package:flutter_bloc/flutter_bloc.dart';
 import 'package:url_launcher/url_launcher.dart';
 
+import '../di/injector.dart';
 import '../i18n/strings.g.dart';
 import '../services/message_store.dart';
 import '../services/profile_cache.dart';
 import '../services/social_service.dart';
 import '../services/social_settings.dart';
+import '../services/unread_service.dart';
 import '../state/messages_cubit.dart';
 import '../state/trust_cubit.dart';
 import 'theme.dart';
@@ -50,9 +52,20 @@ class _ChatScreenState extends State<ChatScreen> {
   String? _peerG1;
   final _input = TextEditingController();
 
+  /// App-wide unread tracker (absent in tests / inventory-only builds). While
+  /// this chat is open it is the "active peer", so incoming messages for it are
+  /// read on arrival and never badge or notify.
+  UnreadService? _unread;
+
   @override
   void initState() {
     super.initState();
+    _unread =
+        getIt.isRegistered<UnreadService>() ? getIt<UnreadService>() : null;
+    // Set synchronously (before any await) so a message landing during _init is
+    // already suppressed.
+    _unread?.activePeer = widget.peerPubkey;
+    unawaited(_unread?.markRead(widget.peerPubkey));
     _init();
   }
 
@@ -148,6 +161,10 @@ class _ChatScreenState extends State<ChatScreen> {
 
   @override
   void dispose() {
+    if (_unread?.activePeer == widget.peerPubkey) _unread!.activePeer = null;
+    // Mark read once more on the way out, so anything that arrived while the
+    // chat was open (and was auto-read) stays cleared.
+    unawaited(_unread?.markRead(widget.peerPubkey));
     _messages?.close();
     _trust?.close();
     _session?.close();