feat: 转正弹窗显示原薪资 + 薪资变化时联动签署

- ConfirmModal 显示原薪资（¥xxx），转正薪资默认填入原薪资
- 转正薪资与原薪资不同时，提示"将发起薪酬调整确认书签署"
- confirmMutation onSuccess：薪资变化时弹出签署方式选择弹窗
  - scene=POLICY，文件标题为"转正薪酬调整确认书"
  - 备注记录薪资变化：¥原薪资 → ¥新薪资

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

frontend/src/pages/roster/modals.tsx

@@ -335,11 +335,15 @@ export function ConfirmModal({ employee, onClose, onSubmit, loading, error }: {
   loading: boolean
   error: any
 }) {
+  const originalSalary = employee.monthlySalary ? Number(employee.monthlySalary) : null
   const [form, setForm] = useState({
     confirmDate: new Date().toISOString().slice(0, 10),
-    regularSalary: '',
+    regularSalary: originalSalary ? String(originalSalary) : '',
   })
 
+  /** 转正薪资与原薪资是否不同 */
+  const salaryChanged = form.regularSalary !== '' && Number(form.regularSalary) !== originalSalary
+
   const handleSubmit = () => {
     onSubmit({
       employeeId: employee.id,
@@ -363,9 +367,15 @@ export function ConfirmModal({ employee, onClose, onSubmit, loading, error }: {
           <Input type="date" value={form.confirmDate} onChange={(e) => setForm({ ...form, confirmDate: e.target.value })} />
         </div>
         <div>
-          <Label>转正薪资（选填）</Label>
-          <Input type="number" value={form.regularSalary} onChange={(e) => setForm({ ...form, regularSalary: e.target.value })} placeholder="不填则保持原薪资" />
-          <div className="text-xs text-gray-400 mt-1">填写后将更新员工月工资并记录薪资变更</div>
+          <Label>转正薪资{originalSalary ? `（原薪资 ¥${originalSalary}）` : ''}</Label>
+          <Input type="number" value={form.regularSalary} onChange={(e) => setForm({ ...form, regularSalary: e.target.value })} placeholder={originalSalary ? `不填则保持原薪资 ¥${originalSalary}` : '请输入转正薪资'} />
+          <div className="text-xs text-gray-400 mt-1">
+            {originalSalary
+              ? (salaryChanged
+                ? <span className="text-amber-600">⚠ 转正薪资与原薪资不同，提交后将发起薪酬调整确认书签署</span>
+                : '填写后将更新员工月工资并记录薪资变更')
+              : '填写后将更新员工月工资并记录薪资变更'}
+          </div>
         </div>
         {error && (
           <div className="text-xs text-danger">