Enable back button on client_app chat screen for iOS

automaticallyImplyLeading was set to false, hiding the back arrow.
iOS has no physical back button so this is needed for navigation.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

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

@@ -107,7 +107,7 @@ class _ChatScreenState extends ConsumerState<ChatScreen> {
     return Scaffold(
       appBar: AppBar(
         title: Text(widget.mitraName),
-        automaticallyImplyLeading: false,
+        automaticallyImplyLeading: true,
         actions: [
           if (chatState is ChatConnectedData && chatState.remainingSeconds != null)
             Padding(