Support changing font family in IDE Extension

What version of the IDE extension are you using?

26.311.21342

What subscription do you have?

Business

Which IDE are you using?

VS Code

What platform is your computer?

Microsoft Windows NT 10.0.26200.0 x64

What issue are you seeing?

The Codex VSCode extension currently appears to use --vscode-font-family for its UI instead of the chat-specific font variable --vscode-chat-font-family.

VSCode provides a dedicated setting:

chat.fontFamily

which is exposed to extensions via the CSS variable:

--vscode-chat-font-family

This variable is used by the built-in Chat UI and by several other chat-based extensions so that users can customize the font used in chat interfaces without affecting the rest of the workbench.

However, the Codex extension seems to rely on --vscode-font-family, and in some cases the variable is redefined inside the webview, which prevents users from customizing the chat font consistently.

What steps can reproduce the bug?

The Codex UI ignores chat.fontFamily, causing the chat interface to use a different font from the rest of the VSCode chat ecosystem.

More specifically, Codex will display Chinese characters incorrectly. This is because the default Segoe UI font will interpret Chinese characters as Japanese or other fonts.

<img width="1585" height="1040" alt="Image" src="https://github.com/user-attachments/assets/85d7d81c-2909-48e3-8fbf-20f89716aab0" />

What is the expected behavior?

Follow the correct font settings, just like Claude Code:

<img width="1586" height="1040" alt="Image" src="https://github.com/user-attachments/assets/e1234437-14a7-42d8-8adb-d7f3e1363576" />

Additional information

_No response_