Add open student registration with email confirmation and approval

Students could previously join by invite only. Add an optional
self-approval mode, toggled from Studio Settings → Registration: anyone
may sign up on the existing [us_student_register] page, confirm their
email via a tokenised link, and then be approved by a studio admin
before the account is usable.

- Enabling the toggle mirrors WordPress's own membership settings
  (users_can_register + default_role = us_student) and snapshots their
  previous values so disabling restores them.
- WordPress's native registration form is blocked while open
  registration is on (login_init redirect + registration_errors
  fail-safe + register_url) so it cannot bypass signup policy acceptance.
- Pending accounts: unconfirmed email cannot log in; confirmed but
  unapproved can log in but the booking capability is withheld and the
  booking page shows an "awaiting approval" screen.
- Approve/reject from Students → Pending Students; reject hard-deletes
  the account so the email is freed to re-apply.
- Invite registration is unchanged; both modes coexist.

Account lifecycle lives in user meta (RegistrationStatus); no new tables.

Closes #63

Co-Authored-By: Claude Opus 4.8 <[email protected]>

src/Plugin.php

@@ -3,8 +3,11 @@ declare(strict_types=1);
 
 namespace Unsupervised\Schedular;
 
+use Unsupervised\Schedular\Auth\EmailConfirmationHandler;
 use Unsupervised\Schedular\Auth\InviteRepository;
 use Unsupervised\Schedular\Auth\LoginPage;
+use Unsupervised\Schedular\Auth\RegistrationLoginGate;
+use Unsupervised\Schedular\Auth\RegistrationMailer;
 use Unsupervised\Schedular\Auth\RegistrationPage;
 use Unsupervised\Schedular\Auth\RoleManager;
 use Unsupervised\Schedular\Booking\BookingPage;
@@ -64,12 +67,16 @@ class Plugin {
 
 		// The shortcode and block wrappers share the same page objects so
 		// front-end output is identical whichever way a page embeds them.
+		$registrationMailer = new RegistrationMailer();
+
 		$bookingPage      = new BookingPage();
 		$loginPage        = new LoginPage();
-		$registrationPage = new RegistrationPage( $invites, $policies, $policyVersions, $acceptances );
+		$registrationPage = new RegistrationPage( $invites, $policies, $policyVersions, $acceptances, $settings, $registrationMailer );
 		$groupClassPage   = new GroupClassPage();
 
 		( new RoleManager() )->register();
+		( new RegistrationLoginGate() )->register();
+		( new EmailConfirmationHandler( $settings, $registrationMailer ) )->register();
 		( new AdminMenu( $availability, $bookings, $offerings, $questions, $policies, $policyVersions, $policyService, $invites, $enrollments, $settings, $paymentRepo, $paymentService, $resolver ) )->register();
 		( new RestRegistrar( $availability, $bookings, $offerings, $questions, $policies, $policyVersions, $policyService, $registrationGate, $enrollments, $paymentService ) )->register();
 		( new ShortcodeRegistrar( $bookingPage, $loginPage, $registrationPage, $groupClassPage ) )->register();