Group class term dates, single-class embed mode, and offering editing

Group class offerings now carry real dates: the add/edit form takes a
start date plus a sessions control (one-off, or weekly for N sessions;
the end date is computed as start + (N-1) weeks via
Offering::weeklyTermEnd). Dates are validated strictly (Y-m-d) and shown
in the offerings list and on the student-facing class card, including
the weekly session count.

[us_group_classes offering="<id>"] (block attribute offeringId, chosen
from a dropdown of active classes fetched from the public offerings
endpoint) restricts the page to a single class so the enrolment flow can
be embedded on a page dedicated to that class; a pinned class that is no
longer offered reports itself closed instead of falling back to the
catalog.

Offerings are now editable from the admin screen: an Edit button
prefills the shared add/edit form and saving posts usc_action=update.
Updates always preserve the original owner and currency, and non-admin
instructors can only load and update their own offerings. The form also
gains the previously missing description field and an Active toggle (the
admin-UI counterpart of the REST is_active flag) so an edit cannot wipe
data the form never collected.

Closes #59

Co-Authored-By: Claude Fable 5 <[email protected]>

assets/js/group-classes.js

@@ -10,6 +10,10 @@
     const errorBox = document.getElementById('us-group-error');
     const { restUrl, nonce } = usScheduler;
 
+    // When the shortcode/block pins a single offering, only that class is
+    // shown, so the page can be embedded alongside a full class description.
+    const singleOfferingId = Number(app.dataset.offering || 0);
+
     function apiFetch(path, options = {}) {
         return fetch(restUrl + path, {
             ...options,
@@ -68,16 +72,39 @@
             </div>`;
     }
 
+    // Parse a Y-m-d date into local time; new Date('Y-m-d') would parse as
+    // UTC midnight and can display as the previous day in western timezones.
+    function formatDate(ymd) {
+        const [y, m, d] = ymd.split('-').map(Number);
+        return new Date(y, m - 1, d).toLocaleDateString(undefined, { year: 'numeric', month: 'short', day: 'numeric' });
+    }
+
+    function termLabel(o) {
+        if (!o.term_start) return '';
+        if (!o.term_end || o.term_end === o.term_start) {
+            return formatDate(o.term_start);
+        }
+        const weekMs = 7 * 24 * 60 * 60 * 1000;
+        const sessions = Math.round((new Date(o.term_end) - new Date(o.term_start)) / weekMs) + 1;
+        return `${formatDate(o.term_start)} – ${formatDate(o.term_end)} (${sessions} weekly sessions)`;
+    }
+
     function renderClasses(offerings) {
-        const groups = offerings.filter((o) => o.kind === 'group_class');
+        let groups = offerings.filter((o) => o.kind === 'group_class');
+        if (singleOfferingId) {
+            groups = groups.filter((o) => Number(o.id) === singleOfferingId);
+        }
         if (!groups.length) {
-            list.innerHTML = '<p>No group classes are open for enrolment right now.</p>';
+            list.innerHTML = singleOfferingId
+                ? '<p>This class is not open for enrolment right now.</p>'
+                : '<p>No group classes are open for enrolment right now.</p>';
             return;
         }
 
         list.innerHTML = groups.map((o) => `
             <div class="us-class">
                 <h3>${escHtml(o.title)}</h3>
+                ${termLabel(o) ? `<p>${escHtml(termLabel(o))}</p>` : ''}
                 ${o.schedule_note ? `<p>${escHtml(o.schedule_note)}</p>` : ''}
                 ${o.description ? `<p>${escHtml(o.description)}</p>` : ''}
                 <p>${escHtml(Number(o.price).toFixed(2))} ${escHtml(o.currency)}</p>