Enterprise login in GitHub Copilot app renders raw CSS instead of authentication UI

### Short summary

When attempting to sign in to the GitHub Copilot desktop/app experience using enterprise login, the authentication screen does not render correctly. Instead of displaying the expected sign-in UI, the app shows what appears to be raw CSS content (e.g., @font-face, .theme-dark, .c-button, etc.) directly on the screen. This prevents completing the login flow.

### Affected version or release

v1.0.9

### Installation context

App installed on Windows Machine

### What happened?

When I attempt to sign in to the app using the enterprise login path, the app just begins showing pure CSS instead of the login flow.

### Steps to reproduce

1. Open the GitHub Copilot app
2. Select Sign in with Enterprise
3. Enter email address
4. Observe the screen after the authentication flow begins

### Expected behavior

I expect the login flow to continue

### Additional context

_No response_

Reproduce on a Windows machine with app v1.0.9 by opening the GitHub Copilot app, selecting Sign in with Enterprise, and entering an email address. Start at the enterprise authentication flow and inspect what is rendered after authentication begins. Done means the expected sign-in UI appears instead of raw CSS and the login flow can continue.

索引モデルが issue の本文から書いたものです。