Prevent showing the login page when the user is already logged in

- Laravel Version: 12.18.0
- Nova Version: 5.7.3
- PHP Version: 8.4.1
- Database Driver & Version: Mysql 8.0.40
- Operating System and Version: MacBook Pro
- Browser type and version: Chrome 137.0.7151.104
- Reproduction Repository: https://gitlab.com/bvk-dev/laravel-nova

### Description:
Hi guys!
I’m using Laravel 12 and Nova 5, Everything is working fine so far, but I ran into an issue:

When a user is already logged in (authenticated) and navigates to the /nova/login URL, instead of redirecting to the dashboard page (like /nova/dashboards/main or the main Nova page), it still shows the login page.

✅ What I expected:
If the user is already logged in and tries to access the login page, they should be redirected to the dashboard page.

❌ What happens instead:
The login form is shown again to the authenticated user.

### Detailed steps to reproduce the issue on a fresh Nova installation:

https://github.com/user-attachments/assets/53cebfec-f038-4ed1-8494-cd5c4e3ddb37

Begin by reproducing the behavior at the /nova/login route using the fresh Nova installation described in the issue. Trace the login entry point and authenticated-user handling, then verify that an authenticated user is redirected to /nova/dashboards/main or the main Nova page instead of seeing the login form.

Written by the indexing model from the issue text.