refactor: 删除 PRE_ONBOARD 显式状态，统一按入职日期判定预入职

预入职不再需要显式设置 PRE_ONBOARD 状态，统一按入职日期自动
判定：入职日期在未来 = 预入职（不进入薪资），入职日期 <= 今天
= 正式在职。

删除内容：
- schema: EmployeeStatus 枚举移除 PRE_ONBOARD
- roster.routes.ts: 移除 PRE_ONBOARD 筛选和 /activate API
- payroll2.routes.ts: 移除 NOT PRE_ONBOARD 排除条件
- modals.tsx: 移除入职状态选择器

保留：按 hireDate > 今天 自动判定预入职的逻辑

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

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

frontend/src/pages/roster/modals.tsx

@@ -705,7 +705,7 @@ export function AddEmployeeModal({ onClose, onSubmit, loading, error }: {
     return {
       name: '', department: '', departmentId: '', position: '', hireDate: todayStr, monthlySalary: '',
       idCardNumber: '', gender: '男' as '男' | '女', femaleWorkerType: '' as '' | 'CADRE' | 'WORKER', phone: '',
-      city: '北京', education: '', status: 'ACTIVE' as 'ACTIVE' | 'PRE_ONBOARD',
+      city: '北京', education: '', status: 'ACTIVE',
       contractType: 'FIXED' as 'FIXED' | 'UNFIXED' | 'UNSIGNED',
       signDate: '', startDate: todayStr, endDate: defaultEndDate,
       contractYears: 3, probationMonths: 0, probationSalary: 0,
@@ -1016,7 +1016,6 @@ export function AddEmployeeModal({ onClose, onSubmit, loading, error }: {
         </div>
         <div className="grid grid-cols-4 gap-4">
           <div><Label>入职日期 *</Label><Input type="date" value={form.hireDate} onChange={(e) => handleHireDateChange(e.target.value)} /></div>
-          <div><Label>入职状态</Label><Select value={form.status} onChange={(e) => setForm({ ...form, status: e.target.value as 'ACTIVE' | 'PRE_ONBOARD' })}><option value="ACTIVE">正式入职</option><option value="PRE_ONBOARD">预入职（不进入薪资）</option></Select></div>
           <div><Label>月工资 *</Label><Input type="number" value={form.monthlySalary} onChange={(e) => setForm({ ...form, monthlySalary: e.target.value })} placeholder="元" /></div>
           <div><Label>手机号</Label><Input value={form.phone} onChange={(e) => {
             const phone = e.target.value.replace(/\D/g, '').slice(0, 11)