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/roster/modals.tsx

@@ -408,6 +408,20 @@ export function RehireModal({ employee, onClose, onSubmit, loading, error }: {
     },
     staleTime: Infinity,
   })
+  // 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 defaultEndDate = (() => {
     const d = new Date()
     d.setFullYear(d.getFullYear() + 3)
@@ -543,7 +557,12 @@ export function RehireModal({ employee, 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>
         <div>
@@ -665,6 +684,20 @@ export function AddEmployeeModal({ onClose, onSubmit, loading, error }: {
     },
     staleTime: Infinity,
   })
+  // 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 defaultEndDate = (() => {
     const d = new Date()
     d.setFullYear(d.getFullYear() + 3)
@@ -872,7 +905,7 @@ export function AddEmployeeModal({ onClose, onSubmit, loading, error }: {
         {/* 基本信息 */}
         <div className="grid grid-cols-4 gap-4">
           <div><Label>姓名 *</Label><Input value={form.name} onChange={(e) => setForm({ ...form, name: e.target.value })} placeholder="员工姓名" /></div>
-          <div><Label>部门 *</Label><Input value={form.department} onChange={(e) => setForm({ ...form, department: e.target.value })} placeholder="如：技术部" /></div>
+          <div><Label>部门 *</Label><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><Label>职务/岗位</Label><Input value={form.position} onChange={(e) => setForm({ ...form, position: e.target.value })} placeholder="如：前端工程师" /></div>
           <div><Label>证件号码 *</Label><Input value={form.idCardNumber} onChange={(e) => handleIdCardChange(e.target.value)} placeholder="18位" maxLength={18} /></div>
           {idCardDuplicate?.exists && (