[Feature Request] Let voice input follow system speech language settings

## Summary

Please add an option for the GitHub Copilot app voice input to follow the operating system's speech input language/settings.

## Problem

On Windows, a user can configure the system speech language to Chinese (for example `zh-CN`) and use Windows voice typing (`Win + H`) successfully. However, the Copilot app's built-in microphone input does not appear to follow the system speech language, so Chinese speech may still be recognized as English or otherwise fail to transcribe correctly.

This creates two separate voice input behaviors:

| Input method | Language source |
| --- | --- |
| Windows voice typing (`Win + H`) | Windows system speech language |
| Copilot app built-in microphone | App/service-specific behavior, with no visible language setting |

## Proposed solution

Add a voice input setting such as:

```text
Voice input language:
- Use system default
- English
- 中文（简体）
- 中文（繁體）
- ...
```

At minimum, `Use system default` should make the Copilot app follow the OS speech input language/settings. This would make setup simpler and more consistent for multilingual users.

## Use case

A user has Windows system speech language configured as Chinese (`zh-CN`) and expects the Copilot app microphone to transcribe Chinese speech without needing a separate app-specific setup or workaround.

Start by reviewing the Copilot app's existing built-in microphone input and its Windows integration, then compare how Windows voice typing obtains the configured speech language. Done means the app offers a system-default voice language option and correctly transcribes speech configured as Chinese, including zh-CN, without a separate workaround.

索引モデルが issue の本文から書いたものです。