feat: 补充公积金支持（员工级别可选）

上海等城市存在补充公积金账户，同一企业内高管可能有但普通员工
没有。新增员工级别补充公积金账户关联：

- Employee 表加 supplementaryHousingAccountId 字段
- BatchEntry 表加 supplementaryHousingEmp/Org 字段
- 薪资计算 calcBatchEntry 支持补充公积金（额外算一笔，纳入
  个税扣除、最低工资保护递延逻辑）
- 新增员工弹窗加补充公积金账户选择器（可选，仅显示
  accountType=SUPPLEMENTARY 的公积金账户）
- 员工档案编辑支持补充公积金账户

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

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

backend/src/services/contract.service.ts

@@ -407,6 +407,7 @@ export async function createEmployee(orgId: string, userId: string, data: any) {
         education: data.education || null,
         position: data.position || null,
         status: data.status || 'ACTIVE',
+        supplementaryHousingAccountId: data.supplementaryHousingAccountId || null,
       },
     })
 
@@ -759,6 +760,7 @@ export async function updateEmployee(orgId: string, id: string, data: any) {
   if (data.education !== undefined) updateData.education = data.education
   if (data.position !== undefined) updateData.position = data.position
   if (data.status !== undefined) updateData.status = data.status
+  if (data.supplementaryHousingAccountId !== undefined) updateData.supplementaryHousingAccountId = data.supplementaryHousingAccountId || null
 
   // 参保城市变更：关闭旧城市在保记录，创建新城市记录
   if (data.city !== undefined && data.city !== employee.city) {