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_en.g.dart

@@ -76,6 +76,7 @@ class Translations with BaseTranslations<AppLocale, Translations> {
 	late final Translations$chatList$en chatList = Translations$chatList$en.internal(_root);
 	late final Translations$chat$en chat = Translations$chat$en.internal(_root);
 	late final Translations$trust$en trust = Translations$trust$en.internal(_root);
+	late final Translations$notifications$en notifications = Translations$notifications$en.internal(_root);
 }
 
 // Path: app
@@ -1401,6 +1402,18 @@ class Translations$trust$en {
 	String get circle => 'In your circle';
 }
 
+// Path: notifications
+class Translations$notifications$en {
+	Translations$notifications$en.internal(this._root);
+
+	final Translations _root; // ignore: unused_field
+
+	// Translations
+
+	/// en: 'New message from {name}'
+	String newMessageFrom({required Object name}) => 'New message from ${name}';
+}
+
 // Path: intro.slides
 class Translations$intro$slides$en {
 	Translations$intro$slides$en.internal(this._root);
@@ -2247,6 +2260,7 @@ extension on Translations {
 			'trust.vouch' => 'I know this person',
 			'trust.vouched' => 'You vouch for them',
 			'trust.circle' => 'In your circle',
+			'notifications.newMessageFrom' => ({required Object name}) => 'New message from ${name}',
 			_ => null,
 		};
 	}