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');

client_app/lib/features/chat/screens/chat_screen.dart

@@ -74,6 +74,14 @@ class _ChatScreenState extends ConsumerState<ChatScreen> {
     _scrollToBottom();
   }
 
+  void _exitChat() {
+    if (context.canPop()) {
+      context.pop();
+    } else {
+      context.go('/home');
+    }
+  }
+
   @override
   Widget build(BuildContext context) {
     final chatState = ref.watch(chatProvider);
@@ -112,7 +120,12 @@ class _ChatScreenState extends ConsumerState<ChatScreen> {
       }
     });
 
-    return Scaffold(
+    return PopScope(
+      canPop: false,
+      onPopInvokedWithResult: (didPop, _) {
+        if (!didPop) _exitChat();
+      },
+      child: Scaffold(
         appBar: AppBar(
           backgroundColor: Colors.white,
           foregroundColor: Colors.black,
@@ -120,7 +133,7 @@ class _ChatScreenState extends ConsumerState<ChatScreen> {
           centerTitle: true,
           leading: IconButton(
             icon: const Icon(Icons.chevron_left, size: 28),
-          onPressed: () => context.pop(),
+            onPressed: _exitChat,
           ),
           title: Text(widget.mitraName),
           actions: [
@@ -140,6 +153,7 @@ class _ChatScreenState extends ConsumerState<ChatScreen> {
           ],
         ),
         body: _buildBody(chatState, closureState),
+      ),
     );
   }