Web: Use AI Features fails because chat setup command is not registered

Type: Bug

## Summary

Copilot setup cannot be started in VS Code Insiders for the Web. There is no GitHub/Copilot sign-in action in the Accounts menu, and selecting **Use AI Features** from the Copilot status menu tries to execute a command that is not registered.

This prevents users who are signed out from enabling AI features in the browser.

## Steps to reproduce

1. Open VS Code Insiders for the Web while signed out of GitHub/Copilot.
2. Open the Accounts menu and look for a GitHub/Copilot sign-in action.
3. Open the Copilot status menu from the Copilot icon in the bottom-right corner.
4. Select **Use AI Features**.

## Actual result

The Accounts menu does not provide a way to sign in to GitHub/Copilot. Selecting **Use AI Features** has no visible effect and logs the following error:

```text
ERR command 'workbench.action.chat.triggerSetup' not found: Error: command 'workbench.action.chat.triggerSetup' not found
at rmt._tryExecuteCommand (workbench.web.main.internal.js:2017:4832)
at rmt.executeCommand (workbench.web.main.internal.js:2017:4732)
```

## Expected result

The browser should offer a working GitHub/Copilot sign-in and setup flow. Selecting **Use AI Features** should start that flow instead of invoking an unregistered command.

## Version

VS Code Insiders for the Web

Commit: `7b7e49c83affacfac726040280da69b4999f3e01`

## Additional information

The same `workbench.action.chat.triggerSetup not found` error was previously reported in #254160, but that issue concerned running Code OSS and Copilot Chat from source and was closed as not planned. This report concerns the published VS Code Insiders web build.

Reproduce the failure in VS Code Insiders for the Web while signed out, then trace the Accounts menu and the Copilot status menu action that invokes workbench.action.chat.triggerSetup. Confirm that the browser provides a GitHub/Copilot sign-in action and that Use AI Features starts the setup flow without the command-not-found error.

Written by the indexing model from the issue text.