feat: 离职/解聘完整功能

1. 已有离职/解聘记录的员工不能再次解聘（后端校验+前端提示）
2. 员工主动离职功能：花名册列表增加「离职」操作按钮和弹窗
3. 离职也按离职日期动态判断在职/离职状态，支持提前办理
4. 员工档案「解聘记录」tab改为「离职/解聘记录」，区分类型显示
5. Schema: TerminationRecord 增加 type、resignationReason 字段
6. 后端新增 POST /termination/resignation 接口
7. 花名册列表返回 hasTermination 标记

frontend/src/pages/Termination.tsx

@@ -26,6 +26,7 @@ interface RosterEmployee {
   name: string
   department: string
   status: string
+  hasTermination?: boolean
   hireDate: string
   monthlySalary: number
   latestContract: any
@@ -276,7 +277,7 @@ export default function Termination() {
   }, [selectedEmployee, terminationDate, socialAvgWage, reason])
 
   const canProceed = () => {
-    if (step === 0) return !!employeeId
+    if (step === 0) return !!employeeId && !selectedEmployee?.hasTermination
     if (step === 1) return !!reason && !!terminationDate && (!riskAssessment?.warnings.length || acknowledgeRisk)
     if (step === 2) return true
     if (step === 3) return true
@@ -440,6 +441,9 @@ export default function Termination() {
                 <div className="font-medium">{selectedEmployee.name}（{selectedEmployee.department}）</div>
                 <div>入职日期：{selectedEmployee.hireDate?.toString().slice(0, 10)}</div>
                 <div>月工资：¥{fmt(selectedEmployee.monthlySalary)}</div>
+                {selectedEmployee.hasTermination && (
+                  <div className="text-danger font-medium mt-1">⚠️ 该员工已有离职/解聘记录，如需再次解聘请先办理重新雇佣</div>
+                )}
                 {selectedEmployee.latestContract ? (
                   <div>合同状态：{selectedEmployee.latestContract.contractType === 'UNSIGNED' ? '未签订' : `签订于 ${selectedEmployee.latestContract.signDate?.slice(0, 10) || '未知'}`}</div>
                 ) : (