refactor(时间选择器): 简化日期选择器组件并修改日期格式

移除van-picker-group包装器，直接使用van-date-picker
修改日期格式为年-月-日，移除时间部分
初始化日期值为2024-01-01和2024-12-31

@@ -183,24 +183,26 @@
 
         <!-- 开始时间选择器 -->
         <van-popup v-model:show="showStartTimePicker" position="bottom">
-            <van-picker-group
+            <van-date-picker
+                v-model="startDate"
                 title="选择开始时间"
+                :min-date="minDate"
+                :max-date="maxDate"
                 @confirm="onStartTimeConfirm"
                 @cancel="showStartTimePicker = false"
-            >
-                <van-date-picker v-model="startDate" :min-date="minDate" :max-date="maxDate" />
-            </van-picker-group>
+            />
         </van-popup>
         
         <!-- 结束时间选择器 -->
         <van-popup v-model:show="showEndTimePicker" position="bottom">
-            <van-picker-group
+            <van-date-picker
+                v-model="endDate"
                 title="选择结束时间"
+                :min-date="minDate"
+                :max-date="maxDate"
                 @confirm="onEndTimeConfirm"
                 @cancel="showEndTimePicker = false"
-            >
-                <van-date-picker v-model="endDate" :min-date="minDate" :max-date="maxDate" />
-            </van-picker-group>
+            />
         </van-popup>
 
         <!-- 课程选择器 -->
@@ -352,8 +354,8 @@ const showClassPicker = ref(false);
 const showGroupPicker = ref(false);
 
 // 日期选择器相关
-const startDate = ref(new Date());
-const endDate = ref(new Date());
+const startDate = ref(['2024', '01', '01']);
+const endDate = ref(['2024', '12', '31']);
 const minDate = new Date(2020, 0, 1);
 const maxDate = new Date(2030, 11, 31);
 
@@ -473,9 +475,7 @@ const formatDateTime = (date) => {
     const year = d.getFullYear();
     const month = String(d.getMonth() + 1).padStart(2, '0');
     const day = String(d.getDate()).padStart(2, '0');
-    const hours = String(d.getHours()).padStart(2, '0');
-    const minutes = String(d.getMinutes()).padStart(2, '0');
-    return `${year}-${month}-${day} ${hours}:${minutes}`;
+    return `${year}-${month}-${day}`;
 };
 
 /**
@@ -500,7 +500,8 @@ const onFrequencyConfirm = (option) => {
  * 开始时间确认
  */
 const onStartTimeConfirm = () => {
-    formData.value.start_time = startDate.value;
+    const [year, month, day] = startDate.value;
+    formData.value.start_time = new Date(parseInt(year), parseInt(month) - 1, parseInt(day));
     showStartTimePicker.value = false;
 };
 
@@ -508,7 +509,8 @@ const onStartTimeConfirm = () => {
  * 结束时间确认
  */
 const onEndTimeConfirm = () => {
-    formData.value.end_time = endDate.value;
+    const [year, month, day] = endDate.value;
+    formData.value.end_time = new Date(parseInt(year), parseInt(month) - 1, parseInt(day));
     showEndTimePicker.value = false;
 };