[GUI] setDefaultModel onChange may cause race with onRefresh

[Vantalens]

File: packages/gui/src/App.tsx:535

Severity: Low

Description:
User rapidly changes default model; setDefaultModel().then(onRefresh) fires but onChange returns immediately; React may batch updates incorrectly or show stale model selection.

Code:

<select value={defaultModel} onChange={(event) => client?.setDefaultModel(event.target.value).then(onRefresh)}>

Failure Scenario:
User changes dropdown from A → B → C rapidly → three API calls fire → refresh callbacks race → UI may briefly show B then settle on C.

Suggested Fix:
Debounce onChange or disable select during update.