ux: 部门字段统一改为下拉选择（组织架构部门树）

将以下页面的部门输入框从 Input 改为 Select 下拉选择，
数据源为组织架构部门树（带层级缩进）：

1. roster/modals.tsx - RehireModal 重新入职弹窗
2. roster/modals.tsx - AddEmployeeModal 添加员工弹窗
3. roster/BasicInfo.tsx - 编辑员工基本信息
4. Contracts.tsx - 合同管理新增员工弹窗

下拉选项从 /departments 接口获取，按树形结构展示，
提交时传部门名称字符串（保持后端兼容）。

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

frontend/src/pages/Contracts.tsx

@@ -4,6 +4,7 @@ import { useQuery, useMutation, useQueryClient } from '@tanstack/react-query'
 import { Plus, Search, Paperclip, Trash2, X, FileText, Download } from 'lucide-react'
 import { toast } from 'sonner'
 import { rosterApi, employeeApi, attachmentApi } from '../lib/api-services'
+import api from '../lib/api'
 import Card from '../components/ui/Card'
 import Button from '../components/ui/Button'
 import { Input, Label, Select } from '../components/ui/Input'
@@ -217,6 +218,20 @@ function AddEmployeeModal({ open, onClose, onSubmit, loading, error }: {
   loading: boolean
   error: any
 }) {
+  // 拉取组织架构部门列表，用于部门下拉选择
+  const { data: departments = [] } = useQuery({
+    queryKey: ['departments'],
+    queryFn: () => api.get('/departments').then(r => r.data),
+  })
+  // 构建部门树形下拉选项（带层级缩进）
+  const deptOptions: { id: string; label: string; level: number }[] = []
+  const buildDeptOptions = (items: any[], parentId: string | null, level: number) => {
+    items.filter(d => d.parentId === parentId).sort((a, b) => a.sortOrder - b.sortOrder).forEach(d => {
+      deptOptions.push({ id: d.id, label: d.name, level })
+      buildDeptOptions(items, d.id, level + 1)
+    })
+  }
+  buildDeptOptions(departments, null, 0)
   const [form, setForm] = useState({
     name: '',
     department: '',
@@ -280,7 +295,12 @@ function AddEmployeeModal({ open, onClose, onSubmit, loading, error }: {
           </div>
           <div>
             <Label>部门 *</Label>
-            <Input value={form.department} onChange={(e) => setForm({ ...form, department: e.target.value })} placeholder="如：技术部" />
+            <Select value={form.department} onChange={(e) => setForm({ ...form, department: e.target.value })}>
+              <option value="">请选择部门</option>
+              {deptOptions.map(d => (
+                <option key={d.id} value={d.label}>{'　'.repeat(d.level)}{d.label}</option>
+              ))}
+            </Select>
           </div>
         </div>