Hide the "Sign up" button when _APP_CONSOLE_WHITELIST_ROOT is set and the first account is created

### 🔖 Enhancement description

The "Sign up" button shown on the login page should be hidden if the `_APP_CONSOLE_WHITELIST_ROOT` variable is set and the first account is already created.

### 🎤 Pitch

This button is confusing as the registration form is accessible but useless. Hiding the button and disabling the route would avoid this confusion.

### 👀 Have you spent some time to check if this issue has been raised before?

- [x] I checked and didn't find similar issue

### 🏢 Have you read the Code of Conduct?

- [x] I have read the [Code of Conduct](https://github.com/appwrite/.github/blob/main/CODE_OF_CONDUCT.md)