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/payroll2.routes.ts

@@ -10,6 +10,7 @@ import {
   generatePayslipFromBatches,
   prePayrollCheck,
 } from '../services/payroll.service'
+import { isInProbation } from '../services/contract.service'
 
 // RFC 5987 编码中文文件名
 function contentDisposition(filename: string): string {
@@ -352,8 +353,12 @@ router.post('/batches', async (req: AuthRequest, res: Response, next: NextFuncti
           where: { employeeId_month: { employeeId: emp.id, month } },
         })
 
-        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
+        // 按批次月份判定是否仍在试用期（试用期结束日 = 合同开始日 + 试用期月数）
+        // 试用期且 probationSalary > 0 → 用试用期工资；否则用转正工资
+        const batchMonthEnd = new Date(`${month}-28T23:59:59`) // 月末近似
+        const latestContract = emp.contracts?.[0]
+        if (isInProbation(latestContract, batchMonthEnd) && latestContract.probationSalary > 0) {
+          baseSalary = latestContract.probationSalary
         } else if (emp.monthlySalary) {
           try { baseSalary = Number(decrypt(emp.monthlySalary)) || 0 } catch { baseSalary = Number(emp.monthlySalary) || 0 }
         }
@@ -607,8 +612,11 @@ router.post('/batches/:batchId/employees', async (req: AuthRequest, res: Respons
       if (!emp) continue
 
       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
+      // 按批次月份判定是否仍在试用期
+      const batchMonthEnd = new Date(`${batch.month}-28T23:59:59`)
+      const latestContract = emp.contracts?.[0]
+      if (isInProbation(latestContract, batchMonthEnd) && latestContract.probationSalary > 0) {
+        baseSalary = latestContract.probationSalary
       } else if (emp.monthlySalary) {
         try { baseSalary = Number(decrypt(emp.monthlySalary)) || 0 } catch { baseSalary = Number(emp.monthlySalary) || 0 }
       }