The admin page role check was failing because better-auth v1.5.5 doesn't
automatically include additionalFields (like 'role') in the session response.
This caused the admin guard to treat all users as non-admin and redirect them.
The fix implements a defensive fallback in the AdminGuard that fetches the
role from the database if it's missing from the session, ensuring that admin
users can access the admin panel while protecting against regression.
Fixes#196
