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]>

templates/admin/settings.php

@@ -15,6 +15,7 @@ if (! defined('ABSPATH')) {
  * @var string $etransferEmail
  * @var float  $hstRate
  * @var bool   $stripeConfigured
+ * @var bool   $openRegistration
  */
 ?>
 <div class="wrap">
@@ -88,6 +89,27 @@ if (! defined('ABSPATH')) {
                 </td>
             </tr>
         </table>
+
+        <h2><?php esc_html_e('Registration', 'unsupervised-schedular'); ?></h2>
+        <table class="form-table">
+            <tr>
+                <th scope="row"><?php esc_html_e('Student sign-up', 'unsupervised-schedular'); ?></th>
+                <td>
+                    <fieldset>
+                        <label>
+                            <input type="checkbox" name="open_registration" value="1" <?php checked($openRegistration); ?>>
+                            <?php esc_html_e('Allow anyone to sign up (email confirmation + admin approval)', 'unsupervised-schedular'); ?>
+                        </label>
+                        <p class="description">
+                            <?php esc_html_e('Off by default — students join by invite only. When on, anyone can register on the student registration page; each new account must confirm its email and then be approved by a studio admin under Students → Pending Students before it can be used.', 'unsupervised-schedular'); ?>
+                        </p>
+                        <p class="description">
+                            <?php esc_html_e('Turning this on also switches on Settings → General → “Anyone can register” and sets the default new-user role to Student. Turning it off restores those two settings to what they were before.', 'unsupervised-schedular'); ?>
+                        </p>
+                    </fieldset>
+                </td>
+            </tr>
+        </table>
         <?php submit_button(esc_html__('Save Settings', 'unsupervised-schedular')); ?>
     </form>
 </div>