Add cancellation cutoff limiting how close to a lesson a student can cancel

Students can no longer cancel their own lesson online once it starts within a
configured window; instructors and studio admins can always cancel.

- Studio default `us_cancellation_cutoff_hours` (stored/computed in hours,
  entered and displayed in days under Studio Settings → Cancellations).
- Optional per-offering override `cancellation_cutoff_hours` (entered in hours);
  blank inherits the studio default, 0 allows anytime cancellation.
- `Booking\CancellationPolicy` resolves the effective window and decides;
  `BookingEndpoint::cancel()` returns a 403 `cancellation_closed` when too late.
  The instructor status endpoint and studio-admin student actions bypass it.

Closes #93

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

templates/admin/offerings.php

@@ -103,6 +103,13 @@ if ($editing && null !== $editing->termStart && null !== $editing->termEnd && $e
                 <th><label for="etransfer_email"><?php esc_html_e('E-transfer email', 'unsupervised-schedular'); ?></label></th>
                 <td><input type="email" name="etransfer_email" id="etransfer_email" class="regular-text" placeholder="<?php esc_attr_e('Overrides the studio default', 'unsupervised-schedular'); ?>" value="<?php echo esc_attr($editing->etransferEmail ?? ''); ?>"></td>
             </tr>
+            <tr>
+                <th><label for="cancellation_cutoff_hours"><?php esc_html_e('Cancellation cutoff (hours)', 'unsupervised-schedular'); ?></label></th>
+                <td>
+                    <input type="number" name="cancellation_cutoff_hours" id="cancellation_cutoff_hours" min="0" step="1" placeholder="<?php esc_attr_e('Uses the studio default', 'unsupervised-schedular'); ?>" value="<?php echo esc_attr(null === ($editing->cancellationCutoffHours ?? null) ? '' : (string) $editing->cancellationCutoffHours); ?>">
+                    <p class="description"><?php esc_html_e('How many hours before a lesson a student may still cancel it. Leave blank to use the studio default; 0 lets students cancel any time.', 'unsupervised-schedular'); ?></p>
+                </td>
+            </tr>
             <tr>
                 <th><?php esc_html_e('Active', 'unsupervised-schedular'); ?></th>
                 <td><label><input type="checkbox" name="is_active" value="1" <?php echo null === $editing || $editing->isActive ? 'checked' : ''; ?>> <?php esc_html_e('Open for registration', 'unsupervised-schedular'); ?></label></td>

templates/admin/settings.php

@@ -16,6 +16,7 @@ if (! defined('ABSPATH')) {
  * @var float  $hstRate
  * @var bool   $stripeConfigured
  * @var bool   $openRegistration
+ * @var float  $cancellationCutoffDays
  */
 ?>
 <div class="wrap">
@@ -90,6 +91,17 @@ if (! defined('ABSPATH')) {
             </tr>
         </table>
 
+        <h2><?php esc_html_e('Cancellations', 'unsupervised-schedular'); ?></h2>
+        <table class="form-table">
+            <tr>
+                <th><label for="cancellation_cutoff_days"><?php esc_html_e('Cancellation cutoff (days)', 'unsupervised-schedular'); ?></label></th>
+                <td>
+                    <input type="number" name="cancellation_cutoff_days" id="cancellation_cutoff_days" class="small-text" min="0" step="0.5" value="<?php echo esc_attr(rtrim(rtrim(number_format($cancellationCutoffDays, 2, '.', ''), '0'), '.')); ?>">
+                    <p class="description"><?php esc_html_e('How far ahead of a lesson a student may still cancel it. Within this window only an instructor can cancel. 0 lets students cancel any time. An offering can override this with its own value.', 'unsupervised-schedular'); ?></p>
+                </td>
+            </tr>
+        </table>
+
         <h2><?php esc_html_e('Registration', 'unsupervised-schedular'); ?></h2>
         <table class="form-table">
             <tr>