Add course duplicate and compact list/roster layout

- POST /api/courses/:id/duplicate deep-copies a course with all units,
  items, and quiz questions; opens the copy immediately for renaming
- Roster changed from broken vertical stack to horizontal chip-pills
  (fixes admin-card label override that was collapsing inline-flex)
- item-row padding reduced for tighter lists throughout admin
- Courses and Kids admin panels now use a 2-column auto-fill grid

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

public/admin.js

@@ -280,6 +280,13 @@ $('#course-meta-form').addEventListener('submit', async (e) => {
   openCourseDetail(currentCourseId);
 });
 
+$('#cd-duplicate').addEventListener('click', async () => {
+  const { id } = await api(`/api/courses/${currentCourseId}/duplicate`, { method: 'POST' });
+  currentCourseId = id;
+  showPanel('course-detail');
+  msg('Course duplicated — update the name and details below.');
+});
+
 $('#cd-delete').addEventListener('click', async () => {
   if (!confirm('Delete this whole course, including all units, items, and grades?')) return;
   await api(`/api/courses/${currentCourseId}`, { method: 'DELETE' });