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

backend/src/routes/import.routes.ts

@@ -124,7 +124,7 @@ router.post('/excel/preview', authMiddleware, requireAdmin, upload.single('file'
       const rows = XLSX.utils.sheet_to_json(empSheet)
       for (let i = 0; i < rows.length; i++) {
         const r = rows[i] as any
-        const row: any = { rowNo: i + 2, name: val(getField(r, '姓名')), department: val(getField(r, '部门')) || '未分配', hireDate: getField(r, '入职日期'), salary: num(getField(r, '月工资')), phone: val(getField(r, '手机号')), idCard: val(getField(r, '身份证号')), city: val(getField(r, '参保城市')) || '北京', status: 'normal', errors: [] as string[], warnings: [] as string[] }
+        const row: any = { rowNo: i + 2, name: val(getField(r, '姓名')), department: val(getField(r, '部门')) || '未分配', hireDate: getField(r, '入职日期'), salary: num(getField(r, '月工资')), phone: val(getField(r, '手机号')), idCard: val(getField(r, '身份证号')), city: val(getField(r, '参保城市')) || null, status: 'normal', errors: [] as string[], warnings: [] as string[] }
         if (!row.name) { row.status = 'error'; row.errors.push('姓名为空') }
         const hireDate = parseDate(getField(r, '入职日期'))
         if (!hireDate) { row.status = 'error'; row.errors.push('入职日期格式错误') }
@@ -285,7 +285,7 @@ router.post('/excel', authMiddleware, requireAdmin, upload.single('file'), async
               socialInsBase: num(getField(r, '社保基数')) || num(salary),
               housingFundBase: num(getField(r, '公积金基数')) || num(salary),
               specialDeduction: num(getField(r, '专项附加扣除')) || 0,
-              city: val(getField(r, '参保城市')) || '北京',
+              city: val(getField(r, '参保城市')) || null,
               isPregnant: val(getField(r, '孕期')) === '是',
               isInMedicalPeriod: val(getField(r, '医疗期')) === '是',
               isWorkInjured: val(getField(r, '工伤')) === '是',