可改进两处小问题

<img width="249" height="126" alt="Image" src="https://github.com/user-attachments/assets/7b099e15-dd36-4e91-89e3-0903270a871d" />

如图，默认可切换模型应该新增Opus 4.5

<img width="1234" height="124" alt="Image" src="https://github.com/user-attachments/assets/f1640d18-0bee-4f37-8741-ecd2f4f066fe" />

会话管理方面可允许删去和fork新会话