[Bug]:设置好的大模型丢失

## QwenPaw Version
win10,2.2.1.desktop
用着用着就报错，说我没设置大模型，明明有的，只能退出去，图2的确没有了大模型，重新选。就是正常使用，没有什么特殊流程，好几次了。

No files, tests, or stack traces are listed in the issue. Reproduce on Win10 with QwenPaw 2.2.1.desktop: configure a large model, use normally, and observe when the app reports no model configured. Start by tracing the startup/loading flow and model-selection persistence path (where selected model config is read/written). Done is a fix where the model remains configured through normal usage and reopening the app does not drop it.

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