Add model name highlighting

### Describe the feature or problem you'd like to solve

Currently, I sometimes inadvertently send simple prompts while an expensive model is still selected without realizing it. This leads to unnecessary costs that could be avoided if the model selection were more visually prominent.

### Proposed solution

I suggest adding a highlighting feature for the model name. By making the selected model name stand out visually, users can more easily notice which model they are using, thereby reducing accidental usage of high-cost models for simple tasks.

### Example prompts or workflows

_No response_

### Additional context

#### Technical Context

At present, it is virtually impossible for users to manually modify or "hack" the styling of the model name within the Copilot CLI TUI. Because the UI is tightly controlled, providing this as a native feature is necessary to allow for any visual distinction.

#### Other

It would be even better if the highlighting style is customizable. For example, allowing users to choose from various formats such as:

- Color changes (text or background)
- Bold text
- Blinking / Flashing effect

Providing these options would allow users to set the level of visibility that best suits their needs.