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/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);
@@ -112,34 +120,40 @@ class _ChatScreenState extends ConsumerState<ChatScreen> {
       }
     });
 
-    return Scaffold(
-      appBar: AppBar(
-        backgroundColor: Colors.white,
-        foregroundColor: Colors.black,
-        elevation: 0.5,
-        centerTitle: true,
-        leading: IconButton(
-          icon: const Icon(Icons.chevron_left, size: 28),
-          onPressed: () => context.pop(),
-        ),
-        title: Text(widget.mitraName),
-        actions: [
-          if (chatState is ChatConnectedData && chatState.remainingSeconds != null)
-            Padding(
-              padding: const EdgeInsets.only(right: 16),
-              child: Center(
-                child: Text(
-                  '${chatState.remainingSeconds}s',
-                  style: TextStyle(
-                    color: chatState.remainingSeconds! < 30 ? Colors.red : Colors.black,
-                    fontWeight: FontWeight.bold,
+    return PopScope(
+      canPop: false,
+      onPopInvokedWithResult: (didPop, _) {
+        if (!didPop) _exitChat();
+      },
+      child: Scaffold(
+        appBar: AppBar(
+          backgroundColor: Colors.white,
+          foregroundColor: Colors.black,
+          elevation: 0.5,
+          centerTitle: true,
+          leading: IconButton(
+            icon: const Icon(Icons.chevron_left, size: 28),
+            onPressed: _exitChat,
+          ),
+          title: Text(widget.mitraName),
+          actions: [
+            if (chatState is ChatConnectedData && chatState.remainingSeconds != null)
+              Padding(
+                padding: const EdgeInsets.only(right: 16),
+                child: Center(
+                  child: Text(
+                    '${chatState.remainingSeconds}s',
+                    style: TextStyle(
+                      color: chatState.remainingSeconds! < 30 ? Colors.red : Colors.black,
+                      fontWeight: FontWeight.bold,
+                    ),
                   ),
                 ),
               ),
-            ),
-        ],
+          ],
+        ),
+        body: _buildBody(chatState, closureState),
       ),
-      body: _buildBody(chatState, closureState),
     );
   }