fix: change calendar v-show to v-if to prevent FC layout break on view switch

FC mis-calculates column widths when switching from hidden (v-show: none)
to visible. Using v-if forces full remount with correct container dimensions.

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

src/pages/clazz/index.vue

@@ -83,7 +83,7 @@
     </div>
 
     <template v-else>
-      <div v-show="viewMode === 'calendar'" class="calendar-wrapper">
+      <div v-if="viewMode === 'calendar'" class="calendar-wrapper">
         <div class="calendar">
           <FullCalendar ref="calendar" :options="options" />
         </div>