Add registration-page selector to the Invites page

Invitation links previously pointed at the site home page, which usually
does not host the [us_student_register] shortcode. Let the studio admin
choose the registration page (stored in the us_registration_page_id
option); invitation links now point there, falling back to the home page
when unset (with a warning notice).

- RegistrationController: OPTION_PAGE constant; set_page action; pass the
  page id/url to the template.
- templates/admin/invites.php: wp_dropdown_pages selector + save; build the
  invite link from the selected page.
- Doc updated.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/features/account-registration.md

@@ -41,6 +41,7 @@ recorded in `us_policy_acceptances` with `registration_type = account` and
 
 ## Admin Interface
 **Invites** in wp-admin (`manage_students`, studio admin only):
+- Select the **registration page** (the page hosting `[us_student_register]`), stored in the `us_registration_page_id` option; invitation links point there (falling back to the home page if unset)
 - Invite an email (creates a pending invite + link)
 - List pending invites; revoke an invite