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/home_screen.dart

@@ -5,6 +5,7 @@ import '../i18n/strings.g.dart';
 import 'app_drawer.dart';
 import 'seed_glyph.dart';
 import 'theme.dart';
+import 'unread_badge.dart';
 
 /// The main menu (redesign screen 00): a sprout logo in a soft green disc over a
 /// faint seed-glyph watermark, with "Your inventory" as the primary green call
@@ -21,7 +22,20 @@ class HomeScreen extends StatelessWidget {
   Widget build(BuildContext context) {
     final t = context.t;
     return Scaffold(
-      appBar: AppBar(title: Text(t.app.title)),
+      appBar: AppBar(
+        title: Text(t.app.title),
+        // Badge the hamburger so unread messages are visible from home, without
+        // opening the drawer. Total count across conversations.
+        leading: Builder(
+          builder: (context) => UnreadBadge(
+            child: IconButton(
+              icon: const Icon(Icons.menu),
+              tooltip: MaterialLocalizations.of(context).openAppDrawerTooltip,
+              onPressed: () => Scaffold.of(context).openDrawer(),
+            ),
+          ),
+        ),
+      ),
       drawer: AppDrawer(marketEnabled: marketEnabled),
       body: Stack(
         children: [