Add group-class withdrawal deadline and kind-aware offering form

Group classes now carry an optional per-class withdrawal deadline. Up to
that day a student may withdraw themselves from the class; the withdrawal
frees the seat and voids any pending payment but never issues an account
credit. After the deadline self-withdrawal closes and a studio admin must
withdraw the student by hand (the admin path is never subject to the
deadline). A blank deadline keeps self-withdrawal open indefinitely.

Also make the Add/Edit Offering form show only the fields relevant to the
selected kind: group settings for group classes, weekly reservation for
private lessons. Progressive enhancement — without JS every field renders.

- New nullable us_offerings.withdrawal_deadline column; Offering model gains
  $withdrawalDeadline + isWithdrawalOpen().
- New student endpoint POST /enrollments/{id}/withdraw, gated by the deadline
  (403 withdrawal_closed), ownership-checked, idempotent.
- Front-end group-class page shows a Withdraw button while open.
- No USC_VERSION bump: 1.2.0 is unreleased and accumulates schema changes
  under its section, matching the scheduled-billing and credit features.

Tests: composer test (596), composer lint, composer cs all pass.

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

assets/js/group-classes.js

@@ -123,7 +123,14 @@
         return !deadline || todayYmd() <= deadline;
     }
 
-    function renderClasses(offerings, enrolledOfferingIds) {
+    // Self-withdrawal closes at the end of the withdrawal-deadline day. Unlike
+    // enrolment there is no implicit default: an unset deadline keeps withdrawal
+    // open. Mirrors the server-side Offering::isWithdrawalOpen() gate.
+    function isWithdrawalOpen(o) {
+        return !o.withdrawal_deadline || todayYmd() <= o.withdrawal_deadline;
+    }
+
+    function renderClasses(offerings, enrolledMap) {
         let groups = offerings.filter((o) => o.kind === 'group_class');
         if (singleOfferingId) {
             groups = groups.filter((o) => Number(o.id) === singleOfferingId);
@@ -143,11 +150,14 @@
                 ${o.schedule_note ? `<p>${escHtml(o.schedule_note)}</p>` : ''}
                 ${o.description ? `<p>${escHtml(o.description)}</p>` : ''}
                 <p>${escHtml(Number(o.price).toFixed(2))} ${escHtml(o.currency)}</p>
-                ${!enrolledOfferingIds.has(Number(o.id)) && isEnrollmentOpen(o) && enrolmentDeadline(o)
+                ${!enrolledMap.has(Number(o.id)) && isEnrollmentOpen(o) && enrolmentDeadline(o)
         ? `<p class="us-enrol-deadline">Enrol by ${escHtml(formatDate(enrolmentDeadline(o)))}</p>`
         : ''}
-                ${enrolledOfferingIds.has(Number(o.id))
-        ? '<p class="us-enrolled"><strong>You are enrolled in this class.</strong></p>'
+                ${enrolledMap.has(Number(o.id))
+        ? `<p class="us-enrolled"><strong>You are enrolled in this class.</strong></p>
+                ${isWithdrawalOpen(o)
+            ? `<button data-enrollment-id="${enrolledMap.get(Number(o.id))}" class="us-withdraw-btn">Withdraw</button>`
+            : '<p class="us-withdraw-closed">Withdrawal has closed — contact the studio to withdraw.</p>'}`
         : (isEnrollmentOpen(o)
             ? `<button data-offering-id="${o.id}" class="us-enrol-btn">Enrol</button>`
             : '<p class="us-enrol-closed"><strong>Enrolment has closed.</strong></p>')}
@@ -158,6 +168,20 @@
             const offering = groups.find((o) => String(o.id) === btn.dataset.offeringId);
             btn.addEventListener('click', () => openEnrolment(offering));
         });
+
+        list.querySelectorAll('.us-withdraw-btn').forEach((btn) => {
+            btn.addEventListener('click', () => withdraw(btn.dataset.enrollmentId));
+        });
+    }
+
+    function withdraw(enrollmentId) {
+        clearError();
+        if (!window.confirm('Withdraw from this class? Your seat is released and any pending payment is cancelled.')) {
+            return;
+        }
+        apiFetch(`enrollments/${enrollmentId}/withdraw`, { method: 'POST' })
+            .then(loadClasses)
+            .catch((err) => showError(err.message));
     }
 
     function openEnrolment(offering) {
@@ -240,9 +264,9 @@
         ])
             .then(([offerings, enrollments]) => renderClasses(
                 offerings,
-                new Set(enrollments
+                new Map(enrollments
                     .filter((e) => e.status === 'active')
-                    .map((e) => Number(e.offering_id)))
+                    .map((e) => [Number(e.offering_id), e.id]))
             ))
             .catch((err) => showError(err.message));
     }