Fix user name preservation in save_checklist - Preserve existing userName from mobile app instead of overriding it

app.py

@@ -591,9 +591,9 @@ async def save_checklist(user_id: str, checklist_data: dict):
         if 'metadata' not in checklist_dict:
             checklist_dict['metadata'] = {}
         
-        # Add user name from metadata if available, or use a default
-        user_name = checklist_dict['metadata'].get('userName', 'Unknown User')
-        checklist_dict['metadata']['userName'] = user_name
+        # Preserve existing userName or set default
+        if 'userName' not in checklist_dict['metadata']:
+            checklist_dict['metadata']['userName'] = 'Unknown User'
         checklist_dict['metadata']['userId'] = user_id
         checklist_dict['metadata']['savedAt'] = datetime.utcnow().isoformat()
         checklist_dict['metadata']['savedAtFormatted'] = datetime.utcnow().strftime('%d/%m/%Y, %H:%M:%S')