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/Booking/BookingPage.php

@@ -3,6 +3,7 @@ declare(strict_types=1);
 
 namespace Unsupervised\Schedular\Booking;
 
+use Unsupervised\Schedular\Auth\RegistrationStatus;
 use Unsupervised\Schedular\Auth\RoleManager;
 use Unsupervised\Schedular\Val;
 
@@ -26,6 +27,10 @@ class BookingPage {
 			);
 		}
 
+		if ( RegistrationStatus::isAwaitingApproval( get_current_user_id() ) ) {
+			return '<p>' . esc_html__( 'Your account is awaiting studio approval. You will be able to book once a studio admin approves it.', 'unsupervised-schedular' ) . '</p>';
+		}
+
 		if ( ! current_user_can( RoleManager::CAP_BOOK_LESSON ) ) {
 			return '<p>' . esc_html__( 'This page is for students only.', 'unsupervised-schedular' ) . '</p>';
 		}