Cannot correct a mistyped GitHub Enterprise domain during setup

## Short summary

GitHub Enterprise sign-in remains locked to a mistyped domain with no way to edit or reset it.

## Affected version or release

v1.1.12 (commit `5265222`)

## Installation context

Local GitHub Copilot desktop app installation on Windows x86-64. The problem occurs during initial setup, before authentication or repository access.

## What happened?

During initial setup, I selected GitHub Enterprise sign-in and mistyped the domain as `ghe.microsoft.com` instead of `microsoft.ghe.com`.

After continuing to the device-code screen, I could not return to the domain-entry step or change the enterprise domain. The only available paths were to perform a health check or return to the device-code screen.

Quitting and reopening the app did not make the domain editable or restart the sign-in configuration, leaving setup blocked.

## Steps to reproduce

1. Install and open the GitHub Copilot app.
2. Begin the initial sign-in experience.
3. Select GitHub Enterprise.
4. Enter an incorrect domain, such as `ghe.microsoft.com`.
5. Continue to the device-code screen.
6. Attempt to return to the domain-entry step and correct the domain.
7. Quit and reopen the app.
8. Observe that there is still no way to enter the correct domain.

## Expected behavior

The sign-in flow should provide a way to go back and edit or clear the GitHub Enterprise domain.

Restarting the app before authentication is completed should also allow the sign-in configuration to be restarted.

## Additional context

- Operating system: Windows x86-64
- GitHub Copilot app: 1.1.12
- App commit: `5265222`
- Bundled Copilot CLI: 1.0.80
- Bundled GitHub CLI: 2.96.0

The exported health check confirms that authentication was not saved:

```json
"auth": {
"host": null,
"login": null,
"message": "No saved GitHub authentication found."
}
```

The application log identifies the affected build:

```text
GitHub Copilot app started ... version=1.1.12 commit=5265222 platform=windows arch=x86_64
```

I have a complete diagnostics archive and can provide it privately if needed. I have not attached it publicly because it contains local usernames, filesystem paths, ports, and environment details.

Reproduce the initial GitHub Enterprise sign-in flow on Windows using a mistyped domain, then inspect the setup state between domain entry and device-code authentication. Confirm that the domain can be edited or cleared and that restarting before authentication resets setup; verify the health check still reports no saved authentication.

索引モデルが issue の本文から書いたものです。