A new option to select the model at the beginning

A new option to select the model will be added when starting a conversation. Currently, there is no way to adjust the model when initiating a dialogue; it can only be done by clicking the "Start" button and entering the flow settings. An additional entry point for switching models should be introduced.

目前开启对话时无法调整模型，只能通过点击“Start”按钮后进入 flow 设置，应新增更换模型入口。