fix: 修复审批和抄送人相关问题

1. 修复抄送人显示问题
   - select-cc.tsx: 添加从后端获取默认抄送人的逻辑
   - 之前只从 sessionStorage 获取，导致默认抄送人无法加载

2. 修复多人审批重复操作问题
   - ApprovalService: approve() 方法添加重复审批检查
   - 检查用户是否已在当前节点审批过，避免重复操作
   - 恢复 isNodeCompleted() 从 form_data 获取审批人列表

3. 审批逻辑优化
   - 从 form_data['approvers'] 获取用户实际选择的审批人
   - 支持会签（and_approve）多人审批场景

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/pages/business-trip/select-cc.tsx

@@ -1,9 +1,9 @@
-import { useState, useEffect } from 'react'
+import { useState, useEffect, useRef } from 'react'
 import { useNavigate, useLocation } from 'react-router-dom'
 import { useTranslation } from 'react-i18next'
 import { NavBar, SearchBar, Button, Toast, Empty } from 'antd-mobile'
 import { CheckOutline } from 'antd-mobile-icons'
-import { userApi } from '../../services/api'
+import { userApi, businessTripApi } from '../../services/api'
 import styles from './select-cc.module.css'
 
 interface UserItem {
@@ -34,16 +34,46 @@ export default function SelectCcPage() {
   const state = location.state as LocationState
 
   const [searchValue, setSearchValue] = useState('')
-  // 只从 sessionStorage 获取对应类型的数据
-  const getInitialSelected = () => {
-    const data = sessionStorage.getItem('selectedCc')
-    return data ? JSON.parse(data) : []
-  }
-  const [selectedUsers, setSelectedUsers] = useState<UserItem[]>(getInitialSelected())
+  const [selectedUsers, setSelectedUsers] = useState<UserItem[]>([])
   const [allUsers, setAllUsers] = useState<UserItem[]>([])
   const [loading, setLoading] = useState(false)
   const [hasMore, setHasMore] = useState(true)
   const [page, setPage] = useState(1)
+  const initialLoadedRef = useRef(false)
+
+  // 获取初始选中的用户（从 sessionStorage 或后端默认配置）
+  const loadInitialSelected = async () => {
+    // 防止重复加载
+    if (initialLoadedRef.current) return
+
+    // 先尝试从 sessionStorage 获取
+    const storedData = sessionStorage.getItem('selectedCc')
+    if (storedData) {
+      try {
+        const parsed = JSON.parse(storedData)
+        setSelectedUsers(parsed)
+        initialLoadedRef.current = true
+        return
+      } catch (e) {
+        console.error('Failed to parse stored data:', e)
+      }
+    }
+
+    // 如果没有存储的数据，从后端获取默认配置
+    try {
+      const defaultData = await businessTripApi.getDefaultApprovers()
+      const defaultUsers = defaultData.cc_users || []
+      setSelectedUsers(defaultUsers)
+      // 将默认数据也保存到 sessionStorage
+      if (defaultUsers.length > 0) {
+        sessionStorage.setItem('selectedCc', JSON.stringify(defaultUsers))
+      }
+    } catch (error: any) {
+      console.error('Failed to load default cc users:', error)
+      setSelectedUsers([])
+    }
+    initialLoadedRef.current = true
+  }
 
   // 加载用户列表
   const loadUsers = async (pageNum: number = 1, isRefresh: boolean = false) => {
@@ -89,6 +119,7 @@ export default function SelectCcPage() {
   // 初始加载
   useEffect(() => {
     loadUsers(1, true)
+    loadInitialSelected()
   }, [])
 
   const isUserSelected = (userId: number) => {