fix: copy_last模式试用期工资被上月工资条覆盖

bug: copy_last模式下试用期判定后被prevPayslip.baseSalary无条件覆盖，
导致试用期员工复制上月批次时仍用转正工资而非probationSalary。

修复: 试用期判定优先，仅非试用期时才用上月工资条baseSalary。

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

backend/src/routes/payroll2.routes.ts

@@ -358,12 +358,15 @@ router.post('/batches', async (req: AuthRequest, res: Response, next: NextFuncti
         // 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) {
+        const inProbation = isInProbation(latestContract, batchMonthEnd)
+        if (inProbation && latestContract.probationSalary > 0) {
           baseSalary = latestContract.probationSalary
+        } else if (prevPayslip) {
+          // 非试用期：优先用上月工资条的基本工资（保持薪资连续性）
+          baseSalary = prevPayslip.baseSalary
         } else if (emp.monthlySalary) {
           try { baseSalary = Number(decrypt(emp.monthlySalary)) || 0 } catch { baseSalary = Number(emp.monthlySalary) || 0 }
         }
-        if (prevPayslip) baseSalary = prevPayslip.baseSalary
         overtimePay = overtime?.totalPay || 0
         allowance = prevPayslip?.allowance || 0
         deduction = prevPayslip?.deduction || 0