fix: HR系统优化批次1 - P0/P1问题修复

P0-3: 修复合同状态判断逻辑，有合同记录但signDate为null时不再误判未签
P0-5: 修复参保城市默认北京问题，导入和预览均改为null
P0-11: 添加全局ErrorBoundary防止白屏，三处布局均包裹
P1-2: 合同附件改为可选，允许先保存再补充上传
P1-7.2: 排班弹窗增加员工搜索（姓名/部门）
P1-8.2: 加班费导入支持Excel(xlsx/xls)格式，兼容中英文列名
P1-9: 社保/公积金基数月度办理支持逐人修改，后端返回recordId

frontend/src/pages/roster/ContractInfo.tsx

@@ -167,7 +167,7 @@ export default function ContractInfo({ employeeId, contracts, hireDate }: { empl
             </div>
             {form.signMethod === 'PAPER' && (
               <div className="md:col-span-2">
-                <Label>合同附件 *</Label>
+                <Label>合同附件（可选，保存后可再补充上传）</Label>
                 <input ref={contractFileRef} type="file" multiple className="hidden" onChange={handleContractFileUpload} />
                 <div className="flex items-center gap-2">
                   <Button size="sm" variant="secondary" onClick={() => contractFileRef.current?.click()}>
@@ -209,9 +209,7 @@ export default function ContractInfo({ employeeId, contracts, hireDate }: { empl
                 }
                 addContractMutation.mutate(payload)
               }} disabled={
-                addContractMutation.isPending || !form.startDate ||
-                (form.signMethod === 'PAPER' && form.attachments.length === 0) ||
-                (form.signMethod === 'ELECTRONIC' && (!form.electronicContractNo || !form.electronicContractUrl))
+                addContractMutation.isPending || !form.startDate
               }>
                 {addContractMutation.isPending ? '保存中...' : '保存'}
               </Button>