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>

src/Auth/RegistrationController.php

@@ -5,6 +5,11 @@ namespace Unsupervised\Schedular\Auth;
 
 class RegistrationController {
 
+	/**
+	 * Option storing the page ID that hosts the [us_student_register] shortcode.
+	 */
+	public const OPTION_PAGE = 'us_registration_page_id';
+
 	public function __construct( private InviteRepository $invites ) {}
 
 	public function renderPage(): void {
@@ -16,7 +21,9 @@ class RegistrationController {
 			$this->handleFormAction();
 		}
 
-		$pendingInvites = $this->invites->findPending();
+		$pendingInvites      = $this->invites->findPending();
+		$registrationPageId  = (int) get_option( self::OPTION_PAGE, 0 );
+		$registrationPageUrl = $registrationPageId > 0 ? (string) get_permalink( $registrationPageId ) : '';
 
 		include USC_PLUGIN_DIR . 'templates/admin/invites.php';
 	}
@@ -26,6 +33,10 @@ class RegistrationController {
 		// phpcs:disable WordPress.Security.NonceVerification.Missing
 		$action = sanitize_key( wp_unslash( $_POST['usc_action'] ?? '' ) );
 
+		if ( 'set_page' === $action ) {
+			update_option( self::OPTION_PAGE, absint( $_POST['registration_page_id'] ?? 0 ) );
+		}
+
 		if ( 'invite' === $action ) {
 			$email = sanitize_email( wp_unslash( $_POST['email'] ?? '' ) );