phsl / new-api

The sign-in route already redirects logged-in users to /dashboard in its
beforeLoad guard, but the sign-up route (and its /register alias) had no
such guard, leaving authenticated users on the registration form. This
mirrors the classic theme's AuthRedirect behavior. Add an equivalent
beforeLoad guard using the same useAuthStore, redirecting to /dashboard
for consistency with the sign-in route.

Closes #5908

Co-authored-by: 贺. <kuang@M1.local>