Let the studio register the students who cannot register themselves

The Book a lesson for a student panel built its picker from the us_student
role but vetted the submission with the book_lesson capability. ChildLoginGate
and RegistrationLoginGate withhold that capability from accounts that keep the
role, so the panel offered every guardian-managed child and every unapproved
signup and then refused them — with a message claiming no student had been
chosen, and a form cleared of all five fields.

Withholding book_lesson stops those accounts registering in their own name. It
was never meant to stop the studio acting for them, which is what the panel is
for, and for a child is the only route to a lesson besides their guardian.

Guard the student role instead, via a new RoleManager::isStudent() shared with
every picker and guard on the staff side so the two cannot drift apart again.
Group enrolment gets the same predicate: addDirect() and grantAccess() vetted
their posted ids not at all, and would enrol an instructor, an administrator,
or an account deleted since the page was drawn — raising a real payment against
them for a priced class.

Keep a refused booking's fields as submitted, reading the form through one
LessonController::submittedBooking() so what gets booked and what is shown
again cannot disagree about a field name. A booking that succeeds still leaves
an empty form, so the next one does not inherit it.

Closes #185

Co-Authored-By: Claude Opus 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01XunBYk2sFEc1oL14sUiuBU

templates/admin/lessons.php

@@ -16,6 +16,7 @@ if (! defined('ABSPATH')) {
  * @var string $notice
  * @var string $error
  * @var array{students: list<array{id: int, name: string}>, offerings: list<array{id: int, label: string}>, slots: list<array{id: int, label: string, weekly: bool}>} $bookForm
+ * @var array{student_id: int, slot_id: int, offering_id: int, weekly: bool, no_charge: bool, notes: string} $bookValues
  */
 ?>
 <div class="wrap">
@@ -54,7 +55,7 @@ if (! defined('ABSPATH')) {
                             <select name="student_id" id="usc-book-student" required>
                                 <option value=""><?php esc_html_e('Choose a student', 'unsupervised-schedular'); ?></option>
                                 <?php foreach ($bookForm['students'] as $student) : ?>
-                                    <option value="<?php echo esc_attr((string) $student['id']); ?>"><?php echo esc_html($student['name']); ?></option>
+                                    <option value="<?php echo esc_attr((string) $student['id']); ?>" <?php selected($bookValues['student_id'], $student['id']); ?>><?php echo esc_html($student['name']); ?></option>
                                 <?php endforeach; ?>
                             </select>
                         </td>
@@ -65,7 +66,7 @@ if (! defined('ABSPATH')) {
                             <select name="slot_id" id="usc-book-slot" required style="max-width:100%;">
                                 <option value=""><?php esc_html_e('Choose an open time', 'unsupervised-schedular'); ?></option>
                                 <?php foreach ($bookForm['slots'] as $slot) : ?>
-                                    <option value="<?php echo esc_attr((string) $slot['id']); ?>"><?php echo esc_html($slot['label']); ?></option>
+                                    <option value="<?php echo esc_attr((string) $slot['id']); ?>" <?php selected($bookValues['slot_id'], $slot['id']); ?>><?php echo esc_html($slot['label']); ?></option>
                                 <?php endforeach; ?>
                             </select>
                         </td>
@@ -76,7 +77,7 @@ if (! defined('ABSPATH')) {
                             <select name="offering_id" id="usc-book-offering" style="max-width:100%;">
                                 <option value="0"><?php esc_html_e('Use the time\'s own lesson type', 'unsupervised-schedular'); ?></option>
                                 <?php foreach ($bookForm['offerings'] as $offering) : ?>
-                                    <option value="<?php echo esc_attr((string) $offering['id']); ?>"><?php echo esc_html($offering['label']); ?></option>
+                                    <option value="<?php echo esc_attr((string) $offering['id']); ?>" <?php selected($bookValues['offering_id'], $offering['id']); ?>><?php echo esc_html($offering['label']); ?></option>
                                 <?php endforeach; ?>
                             </select>
                             <p class="description"><?php esc_html_e('A time already tied to a lesson type is booked as that type; a general time needs one chosen here.', 'unsupervised-schedular'); ?></p>
@@ -86,12 +87,12 @@ if (! defined('ABSPATH')) {
                         <th scope="row"><?php esc_html_e('Options', 'unsupervised-schedular'); ?></th>
                         <td>
                             <label>
-                                <input type="checkbox" name="recurrence_weekly" value="1">
+                                <input type="checkbox" name="recurrence_weekly" value="1" <?php checked($bookValues['weekly']); ?>>
                                 <?php esc_html_e('Reserve this time weekly for the rest of the term', 'unsupervised-schedular'); ?>
                             </label>
                             <p class="description"><?php esc_html_e('Only for a time that repeats weekly. Billed upfront as one payment.', 'unsupervised-schedular'); ?></p>
                             <label>
-                                <input type="checkbox" name="no_charge" value="1">
+                                <input type="checkbox" name="no_charge" value="1" <?php checked($bookValues['no_charge']); ?>>
                                 <?php esc_html_e('No charge — book it free and confirm it now', 'unsupervised-schedular'); ?>
                             </label>
                             <p class="description"><?php esc_html_e('For a make-up or goodwill lesson. Otherwise a pending payment is raised at the lesson type\'s price.', 'unsupervised-schedular'); ?></p>
@@ -99,7 +100,7 @@ if (! defined('ABSPATH')) {
                     </tr>
                     <tr>
                         <th scope="row"><label for="usc-book-notes"><?php esc_html_e('Notes', 'unsupervised-schedular'); ?></label></th>
-                        <td><input type="text" name="notes" id="usc-book-notes" class="regular-text" maxlength="500"></td>
+                        <td><input type="text" name="notes" id="usc-book-notes" class="regular-text" maxlength="500" value="<?php echo esc_attr($bookValues['notes']); ?>"></td>
                     </tr>
                 </table>
                 <p>