style(teacher/checkinPage): 优化日期选择器样式，使用更现代的UI设计

 <!--
   * @Date: 2025-05-29 15:34:17
  * @LastEditors: hookehuyr hookehuyr@gmail.com
- * @LastEditTime: 2025-12-13 21:19:36
+ * @LastEditTime: 2025-12-14 01:28:56
  * @FilePath: /mlaj/src/views/teacher/checkinPage.vue
   * @Description: 文件描述
 -->
@@ -18,9 +18,15 @@
         <CourseGroupCascader @change="on_cascade_change" />
 
         <!-- 日期选择触发区域 -->
-        <div class="date-trigger" @click="showCalendar = true">
-          <div class="current-date">{{ formattedSelectedDate }}</div>
-          <div class="date-hint">点击切换日期</div>
+        <div class="bg-white px-4 py-3 flex items-center justify-between border-b border-gray-100 cursor-pointer" @click="showCalendar = true">
+          <div class="flex items-center gap-2">
+            <van-icon name="calendar-o" class="text-primary text-xl" />
+            <span class="text-base font-bold text-gray-800">{{ formattedSelectedDate }}</span>
+          </div>
+          <div class="flex items-center text-gray-400 text-sm">
+            <span>切换日期</span>
+            <van-icon name="arrow-down" class="ml-1" />
+          </div>
         </div>
       </van-sticky>
 
@@ -720,30 +726,7 @@ const handleAdd = (type) => {
   background-color: #4caf50;
 }
 
-.date-trigger {
-  background-color: #fff;
-  padding: 0.5rem;
-  text-align: center;
-  cursor: pointer;
-  border-bottom: 1px solid #eee;
-  display: flex;
-  align-items: center;
-  justify-content: center;
-  // gap: 0.5rem;
-  flex-direction: column;
-
-  .current-date {
-    font-size: 1rem;
-    font-weight: bold;
-    color: #333;
-    margin-bottom: 0;
-  }
 
-  .date-hint {
-    font-size: 0.8rem;
-    color: #999;
-  }
-}
 
 /* 下拉菜单选项文字单行显示 */
 .van-dropdown-item {