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/app_drawer.dart

@@ -5,6 +5,7 @@ import 'package:material_symbols_icons/symbols.dart';
 import '../i18n/strings.g.dart';
 import 'seed_glyph.dart';
 import 'theme.dart';
+import 'unread_badge.dart';
 
 /// The app's navigation drawer (redesign screen 05). A white sheet: Inventory is
 /// the live destination (green seed glyph), the social items (market, profile,
@@ -56,7 +57,7 @@ class AppDrawer extends StatelessWidget {
                   : null,
             ),
             _DrawerItem(
-              icon: const Icon(Icons.chat_bubble),
+              icon: const UnreadBadge(child: Icon(Icons.chat_bubble)),
               label: t.menu.chat,
               onTap: marketEnabled
                   ? () {