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/pubspec.yaml

@@ -74,6 +74,10 @@ dependencies:
   # Network reachability (BSD-3) to show a global "you're offline" banner — the
   # social layer needs a connection; the inventory works regardless.
   connectivity_plus: ^6.1.0
+  # Local OS notifications (BSD-3) for a heads-up when a private message arrives
+  # while the app is foregrounded. Mobile/Linux/macOS only; a no-op on web and
+  # Windows. Foreground-only by design — background/push is a later concern.
+  flutter_local_notifications: ^18.0.1
 
 dev_dependencies:
   flutter_test: