iOS navigation fixes: deep-link pop fallback + back-button PopScope

- notification_service: use GoRouter.go (not push) for terminal states
  (session_closing, session_expired) so the nav stack doesn't linger
  behind deep-linked screens
- chat_screen: PopScope + canPop fallback in client_app so iOS back
  gestures fall back to /home when there is nothing to pop

client_app/lib/core/notifications/notification_service.dart

@@ -89,9 +89,8 @@ class NotificationService {
     final type = data['type'] as String?;
 
     if (type == 'session_closing' || type == 'session_expired') {
-      // Navigate to the chat session — closure UI will show
       if (sessionId != null) {
-        _router!.push('/chat/session/$sessionId', extra: 'Bestie');
+        _router!.go('/chat/session/$sessionId', extra: 'Bestie');
       }
     } else if ((type == 'chat_message' || type == 'paired') && sessionId != null) {
       _router!.push('/chat/session/$sessionId', extra: 'Bestie');