fix: FullCalendar getApi null 引用保护

draw()/redraw()/freeze() 在组件卸载后可能被异步触发，
添加 calendar.value null 检查防止 TypeError。

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/pages/clazz/index.vue

@@ -625,6 +625,7 @@ export default defineComponent({
     }
 
     const redraw = () => {
+      if (!calendar.value) { console.debug('[clazz redraw] skip, calendar null'); return; }
       let events = calendar.value.getApi().getEvents();
       events.forEach((event, i) => {
         let subsidiary_id = event.extendedProps.subsidiary_id;
@@ -894,6 +895,7 @@ export default defineComponent({
     }
 
     const freeze = () => {
+      if (!calendar.value) return;
       store.hanging = true;
       let date = calendar.value.getApi().getDate();
       setKey('clazz_state', {state, date})