Fix session update in app.py by using update_session instead of save_session

app.py

@@ -138,7 +138,7 @@ if send_button and user_input.strip():
                 
                 # Update session
                 user_session["conversation"] = conversation
-                session_manager.save_session("default_user", user_session)
+                session_manager.update_session("default_user", user_session)
                 
                 # Add assistant response to history
                 st.session_state.messages.append({"role": "assistant", "content": ai_response})