Force all interactions to use the selected model

Force all interactions to use the selected model

Add an option to globally enforce the user's selected model across all interactions, including named and unnamed sub-agents. This would ensure a consistent model experience and prevent background delegation to more expensive or unintended models.

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| Field | Value |
| --- | --- |
| App version | 1.1.8 |
| OS | Windows 10.0.26100 |
| Theme | Rainglow Codecourse |
| Path | /chat |
| Tenure | Week 9 |

从 /chat 入口开始，跟踪所选模型如何应用于交互，包括有名称和无名称的子代理。完成的标准是：某个选项可以全局强制使用用户选择的模型，并防止在这些交互中发生意外的模型委派。

由索引模型根据 Issue 内容生成。