优化: 合同开始日期随入职日期自动同步

1. 添加员工和重新入职：入职日期变更时自动同步合同开始日期和结束日期
2. 合同开始日期改为只读显示（始终等于入职日期）
3. 花名册列表增加离职日期列：已离职显示日期，待离职显示预计日期
4. 重新入职弹窗对齐添加员工：移除孕期/医疗期/工伤，合同双向计算，试用期校验
5. 后端 rehireEmployee 重置特殊状态为 false

frontend/src/pages/Roster.tsx

@@ -879,6 +879,19 @@ function RehireModal({ employee, onClose, onSubmit, loading, error }: {
     setForm({ ...form, endDate, contractYears: Math.max(1, Math.round(months / 12)) })
   }
 
+  // 入职日期变更 → 同步合同开始日期 + 重算结束日期
+  const handleHireDateChange = (hireDate: string) => {
+    if (form.contractType === 'FIXED' && form.contractYears > 0 && hireDate) {
+      const start = new Date(hireDate)
+      const end = new Date(start)
+      end.setFullYear(end.getFullYear() + form.contractYears)
+      end.setDate(end.getDate() - 1)
+      setForm({ ...form, hireDate, startDate: hireDate, endDate: end.toISOString().slice(0, 10) })
+    } else {
+      setForm({ ...form, hireDate, startDate: hireDate })
+    }
+  }
+
   // 开始日期变更 → 重新计算结束日期
   const handleStartDateChange = (startDate: string) => {
     if (form.contractType === 'FIXED' && form.contractYears > 0 && startDate) {
@@ -932,7 +945,7 @@ function RehireModal({ employee, onClose, onSubmit, loading, error }: {
         </div>
         <div>
           <Label>新入职日期 *</Label>
-          <Input type="date" value={form.hireDate} onChange={(e) => setForm({ ...form, hireDate: e.target.value })} />
+          <Input type="date" value={form.hireDate} onChange={(e) => handleHireDateChange(e.target.value)} />
         </div>
         <div className="border-t pt-3">
           <Label>合同类型</Label>
@@ -950,7 +963,7 @@ function RehireModal({ employee, onClose, onSubmit, loading, error }: {
                 <Input type="date" value={form.signDate} onChange={(e) => setForm({ ...form, signDate: e.target.value })} />
                 <div className="text-xs text-gray-400 mt-0.5">留空表示尚未签订</div>
               </div>
-              <div><Label>合同开始日期 *</Label><Input type="date" value={form.startDate} onChange={(e) => handleStartDateChange(e.target.value)} /></div>
+              <div><Label>合同开始日期</Label><div className="text-xs text-gray-600 py-1.5">{form.startDate || '随入职日期'}</div></div>
             </div>
             {form.contractType === 'FIXED' && (
               <>
@@ -1022,6 +1035,19 @@ function AddEmployeeModal({ onClose, onSubmit, loading, error }: {
     contractYears: 3, probationMonths: 0, probationSalary: 0,
   })
 
+  // 入职日期变更 → 同步合同开始日期 + 重算结束日期
+  const handleHireDateChange = (hireDate: string) => {
+    if (form.contractType === 'FIXED' && form.contractYears > 0 && hireDate) {
+      const start = new Date(hireDate)
+      const end = new Date(start)
+      end.setFullYear(end.getFullYear() + form.contractYears)
+      end.setDate(end.getDate() - 1)
+      setForm({ ...form, hireDate, startDate: hireDate, endDate: end.toISOString().slice(0, 10) })
+    } else {
+      setForm({ ...form, hireDate, startDate: hireDate })
+    }
+  }
+
   // 根据身份证号自动计算性别（第17位：奇数=男，偶数=女）
   const handleIdCardChange = (idCard: string) => {
     let gender = form.gender
@@ -1148,7 +1174,7 @@ function AddEmployeeModal({ onClose, onSubmit, loading, error }: {
           <div><Label>性别</Label><div className="text-xs text-gray-600 py-1.5">{form.idCardNumber.length >= 17 ? form.gender : '由身份证号自动识别'}</div></div>
         </div>
         <div className="grid grid-cols-2 gap-3">
-          <div><Label>入职日期 *</Label><Input type="date" value={form.hireDate} onChange={(e) => setForm({ ...form, hireDate: e.target.value })} /></div>
+          <div><Label>入职日期 *</Label><Input type="date" value={form.hireDate} onChange={(e) => handleHireDateChange(e.target.value)} /></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>
         <div className="grid grid-cols-2 gap-3">
@@ -1168,7 +1194,7 @@ function AddEmployeeModal({ onClose, onSubmit, loading, error }: {
                 <Input type="date" value={form.signDate} onChange={(e) => setForm({ ...form, signDate: e.target.value })} />
                 <div className="text-xs text-gray-400 mt-0.5">留空表示尚未签订</div>
               </div>
-              <div><Label>合同开始日期 *</Label><Input type="date" value={form.startDate} onChange={(e) => handleStartDateChange(e.target.value)} /></div>
+              <div><Label>合同开始日期</Label><div className="text-xs text-gray-600 py-1.5">{form.startDate || '随入职日期'}</div></div>
             </div>
             {form.contractType === 'FIXED' && (
               <>