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]>

tests/bootstrap.php

@@ -104,5 +104,13 @@ if (! class_exists('WP_Error')) {
             $code = $this->get_error_code();
             return '' !== $code ? ($this->errors[$code][0] ?? '') : '';
         }
+
+        public function add(string $code, string $message = '', mixed $data = ''): void
+        {
+            $this->errors[$code][] = $message;
+            if ('' !== $data) {
+                $this->error_data[$code] = $data;
+            }
+        }
     }
 }