feat: 待签合同支持登记线下签署日期

- 线下手签合同：展开后可点击「登记签署日期」选择日期并保存
  保存后合同从待签列表移除（signDate 已填写）
- 电子签合同：签署日期由电签系统自动回写，不可手动修改
- 后端新增 POST /esign/sign-date 接口
  电子签合同拒绝手动修改签署日期

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

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

backend/src/routes/payroll.routes.ts

@@ -3,6 +3,7 @@ import prisma from '../lib/prisma'
 import { decrypt } from '../lib/crypto'
 import { authMiddleware, AuthRequest } from '../middleware/auth'
 import { z } from 'zod'
+import { isInProbation } from '../services/contract.service'
 
 const router = Router()
 router.use(authMiddleware)
@@ -377,8 +378,11 @@ router.post('/payslip/batch-generate', async (req: AuthRequest, res: Response, n
       const deduction = deductions[emp.id] || 0
 
       let baseSalary = 0
-      if (emp.contracts[0]?.probationSalary && new Date(emp.contracts[0].startDate) > new Date(Date.now() - 365 * 24 * 60 * 60 * 1000)) {
-        baseSalary = emp.contracts[0].probationSalary
+      // 按月份判定是否仍在试用期
+      const monthEnd = new Date(`${month}-28T23:59:59`)
+      const latestContract = emp.contracts[0]
+      if (isInProbation(latestContract, monthEnd) && latestContract?.probationSalary > 0) {
+        baseSalary = latestContract.probationSalary
       } else if (emp.monthlySalary) {
         try {
           baseSalary = Number(decrypt(emp.monthlySalary)) || 0