fix: 修复日期选择器显示NaN问题

- detail.vue: 修正onDateChange方法，使用e.detail.value获取日期值
- 原生picker返回的是日期字符串，无需Date解析

pages/detail/detail.vue

@@ -580,12 +580,9 @@ const resetTransactionForm = () => {
 };
 
 const onDateChange = (e) => {
-  const date = new Date(e.value);
-  const year = date.getFullYear();
-  const month = String(date.getMonth() + 1).padStart(2, '0');
-  const day = String(date.getDate()).padStart(2, '0');
-  transactionForm.value.transactionDate = `${year}-${month}-${day}`;
-  transactionForm.value.dateTimestamp = e.value;
+  // 原生 picker 直接返回日期字符串 YYYY-MM-DD
+  transactionForm.value.transactionDate = e.detail.value;
+  transactionForm.value.dateTimestamp = new Date(e.detail.value).getTime();
 };
 
 const submitTransaction = async () => {