Fix field-length saves, student wp-admin access, and empty instructor picker

Three bug fixes for the 1.2.1 section:

- Fixed-size fields (question labels, offering titles/notes/e-transfer
  email, policy titles/slugs) no longer silently fail to save when the
  value exceeds its column length. The REST endpoints reject over-long
  values with a 400, the admin controllers refuse to insert them, and the
  form inputs carry a maxlength so the browser blocks over-long entry.
  Limits are MAX_* constants on the value objects, kept in lockstep with
  the schema columns.

- Students are kept out of wp-admin entirely. New StudentAdminGuard
  redirects front-end-only users (no back-office capability) away from the
  dashboard and hides the admin bar for them, while administrators, studio
  admins, and instructors keep full access.

- The Add/Edit Offering instructor picker now includes WordPress
  administrators when they act as instructors (the default single-account
  setup), so a solo studio owner is selectable instead of the dropdown
  being empty.

composer test (618), composer lint, composer cs all pass.

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

templates/admin/offerings.php

@@ -45,7 +45,7 @@ if ($editing && null !== $editing->termStart && null !== $editing->termEnd && $e
         <table class="form-table">
             <tr>
                 <th><label for="title"><?php esc_html_e('Title', 'unsupervised-schedular'); ?></label></th>
-                <td><input type="text" name="title" id="title" class="regular-text" required value="<?php echo esc_attr($editing->title ?? ''); ?>"></td>
+                <td><input type="text" name="title" id="title" class="regular-text" maxlength="<?php echo esc_attr((string) Offering::MAX_TITLE_LENGTH); ?>" required value="<?php echo esc_attr($editing->title ?? ''); ?>"></td>
             </tr>
             <tr>
                 <th><label for="kind"><?php esc_html_e('Kind', 'unsupervised-schedular'); ?></label></th>
@@ -140,11 +140,11 @@ if ($editing && null !== $editing->termStart && null !== $editing->termEnd && $e
             </tr>
             <tr class="us-group-only">
                 <th><label for="schedule_note"><?php esc_html_e('Schedule note', 'unsupervised-schedular'); ?></label></th>
-                <td><input type="text" name="schedule_note" id="schedule_note" class="regular-text" placeholder="<?php esc_attr_e('e.g. Tuesdays 4:00pm', 'unsupervised-schedular'); ?>" value="<?php echo esc_attr($editing->scheduleNote ?? ''); ?>"></td>
+                <td><input type="text" name="schedule_note" id="schedule_note" class="regular-text" maxlength="<?php echo esc_attr((string) Offering::MAX_SCHEDULE_NOTE_LENGTH); ?>" placeholder="<?php esc_attr_e('e.g. Tuesdays 4:00pm', 'unsupervised-schedular'); ?>" value="<?php echo esc_attr($editing->scheduleNote ?? ''); ?>"></td>
             </tr>
             <tr>
                 <th><label for="etransfer_email"><?php esc_html_e('E-transfer email', 'unsupervised-schedular'); ?></label></th>
-                <td><input type="email" name="etransfer_email" id="etransfer_email" class="regular-text" placeholder="<?php esc_attr_e('Overrides the studio default', 'unsupervised-schedular'); ?>" value="<?php echo esc_attr($editing->etransferEmail ?? ''); ?>"></td>
+                <td><input type="email" name="etransfer_email" id="etransfer_email" class="regular-text" maxlength="<?php echo esc_attr((string) Offering::MAX_ETRANSFER_EMAIL_LENGTH); ?>" placeholder="<?php esc_attr_e('Overrides the studio default', 'unsupervised-schedular'); ?>" value="<?php echo esc_attr($editing->etransferEmail ?? ''); ?>"></td>
             </tr>
             <tr>
                 <th><label for="cancellation_cutoff_hours"><?php esc_html_e('Cancellation cutoff (hours)', 'unsupervised-schedular'); ?></label></th>