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.

pubspec.lock

@@ -435,6 +435,30 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "6.0.0"
+  flutter_local_notifications:
+    dependency: transitive
+    description:
+      name: flutter_local_notifications
+      sha256: ef41ae901e7529e52934feba19ed82827b11baa67336829564aeab3129460610
+      url: "https://pub.dev"
+    source: hosted
+    version: "18.0.1"
+  flutter_local_notifications_linux:
+    dependency: transitive
+    description:
+      name: flutter_local_notifications_linux
+      sha256: "8f685642876742c941b29c32030f6f4f6dacd0e4eaecb3efbb187d6a3812ca01"
+      url: "https://pub.dev"
+    source: hosted
+    version: "5.0.0"
+  flutter_local_notifications_platform_interface:
+    dependency: transitive
+    description:
+      name: flutter_local_notifications_platform_interface
+      sha256: "6c5b83c86bf819cdb177a9247a3722067dd8cc6313827ce7c77a4b238a26fd52"
+      url: "https://pub.dev"
+    source: hosted
+    version: "8.0.0"
   flutter_localizations:
     dependency: transitive
     description: flutter
@@ -1305,6 +1329,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "0.6.16"
+  timezone:
+    dependency: transitive
+    description:
+      name: timezone
+      sha256: dd14a3b83cfd7cb19e7888f1cbc20f258b8d71b54c06f79ac585f14093a287d1
+      url: "https://pub.dev"
+    source: hosted
+    version: "0.10.1"
   typed_data:
     dependency: transitive
     description: