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.html

@@ -76,6 +76,7 @@
             <input id="cd-color-input" type="color">
             <label class="checkbox-label"><input type="checkbox" id="cd-archived"> Archived</label>
             <button type="submit">Save</button>
+            <button type="button" id="cd-duplicate" class="secondary">Duplicate course</button>
             <button type="button" id="cd-delete" class="danger">Delete course</button>
           </form>
         </div>