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/android/app/src/main/AndroidManifest.xml

@@ -2,6 +2,9 @@
     <!-- Coarse location only, for the optional "use my location" sharing area
          (reduced to a low-precision geohash; never a precise fix). -->
     <uses-permission android:name="android.permission.ACCESS_COARSE_LOCATION" />
+    <!-- Foreground local notifications for incoming private messages (Android 13+
+         asks the user at runtime). -->
+    <uses-permission android:name="android.permission.POST_NOTIFICATIONS" />
     <application
         android:label="tane"
         android:name="${applicationName}"