fix: 请假表单日期序列化与错误消息格式化修复

- el-date-picker 添加 value-format="YYYY-MM-DD"，确保日期以字符串格式发送而非 ISO datetime，修复后端 Pydantic date 校验 422 错误
- 错误处理中正确解析 FastAPI 422 返回的 detail 数组，提取 msg 字段拼接显示，修复 [object Object] 显示问题

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

frontend/src/views/desktop/Leaves.vue

@@ -138,7 +138,9 @@ async function handleSave() {
     dialogVisible.value = false
     await loadLeaves()
   } catch (e: any) {
-    ElMessage.error('保存失败: ' + (e.response?.data?.detail || e.message))
+    const detail = e.response?.data?.detail
+    const msg = Array.isArray(detail) ? detail.map((d: any) => d.msg).join('; ') : (detail || e.message)
+    ElMessage.error('保存失败: ' + msg)
   }
 }
 
@@ -276,12 +278,14 @@ function onStatusChange(val: string) {
             type="date"
             placeholder="开始日期"
             style="width:100%; margin-bottom: 8px"
+            value-format="YYYY-MM-DD"
           />
           <el-date-picker
             v-model="form.end_date"
             type="date"
             placeholder="结束日期"
             style="width:100%"
+            value-format="YYYY-MM-DD"
           />
         </el-form-item>
         <el-form-item label="原因（选填）">