[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.

Beginne damit, die vorhandene integrierte Mikrofoneingabe der Copilot-App und ihre Windows-Integration zu überprüfen, und vergleiche anschließend, wie Windows voice typing die konfigurierte Sprache für die Spracherkennung abruft. Als erledigt gilt die Aufgabe, wenn die App eine Option für die standardmäßige Systemsprache der Spracheingabe anbietet und Sprache, die als Chinesisch konfiguriert ist, einschließlich zh-CN, korrekt transkribiert, ohne eine separate Umgehungslösung.

Vom Indexierungsmodell aus dem Issue-Text verfasst.