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/i18n/strings_ast.g.dart

@@ -75,6 +75,7 @@ class TranslationsAst extends Translations with BaseTranslations<AppLocale, Tran
 	@override late final _Translations$chatList$ast chatList = _Translations$chatList$ast._(_root);
 	@override late final _Translations$chat$ast chat = _Translations$chat$ast._(_root);
 	@override late final _Translations$trust$ast trust = _Translations$trust$ast._(_root);
+	@override late final _Translations$notifications$ast notifications = _Translations$notifications$ast._(_root);
 }
 
 // Path: app
@@ -756,6 +757,16 @@ class _Translations$trust$ast extends Translations$trust$en {
 	@override String get circle => 'Nel to círculu';
 }
 
+// Path: notifications
+class _Translations$notifications$ast extends Translations$notifications$en {
+	_Translations$notifications$ast._(TranslationsAst root) : this._root = root, super.internal(root);
+
+	final TranslationsAst _root; // ignore: unused_field
+
+	// Translations
+	@override String newMessageFrom({required Object name}) => 'Mensaxe nuevu de ${name}';
+}
+
 // Path: intro.slides
 class _Translations$intro$slides$ast extends Translations$intro$slides$en {
 	_Translations$intro$slides$ast._(TranslationsAst root) : this._root = root, super.internal(root);
@@ -1483,6 +1494,7 @@ extension on TranslationsAst {
 			'trust.vouch' => 'Conozo a esta persona',
 			'trust.vouched' => 'Avales a esta persona',
 			'trust.circle' => 'Nel to círculu',
+			'notifications.newMessageFrom' => ({required Object name}) => 'Mensaxe nuevu de ${name}',
 			_ => null,
 		};
 	}