refactor(server): 日期/月份格式化统一改用 dayjs

- 替换散落的 toISOString().slice(0,10) / split('T')[0] / replace('T',' ')（UTC 语义用 dayjs(...).utc() 保持完全一致）
- Asia/Shanghai 固定时区日期（Intl.DateTimeFormat en-CA）改用 dayjs().utcOffset(8)
- 月份边界 first/last、daysInMonth、nextMonth、shiftDate/addDays 等改 dayjs 简洁实现
- 涉及 attendance/classes/classrooms/room/dashboard/bills/expenses/occupancies/sync/ai-chat/rental 等 28 个文件

apps/server/src/occupancies/occupancy-operations.service.ts

@@ -6,6 +6,7 @@ import { Room } from '../entities/room.entity';
 import { Student } from '../entities/student.entity';
 import { Bed } from '../entities/bed.entity';
 import { Locker } from '../entities/locker.entity';
+import dayjs from '../common/dayjs';
 import { Deposit } from '../entities/deposit.entity';
 import { Organization } from '../entities/organization.entity';
 import { RoomInspectionDetail } from '../entities/room-inspection-detail.entity';
@@ -158,7 +159,7 @@ export class OccupancyOperationsService {
       const transferDate = new Date(dto.transferDate);
       const nextDay = new Date(transferDate);
       nextDay.setDate(nextDay.getDate() + 1);
-      const defaultBillingStart = nextDay.toISOString().split('T')[0];
+      const defaultBillingStart = dayjs(nextDay).utc().format('YYYY-MM-DD');
       this.assertDateOrder(
         dto.transferDate,
         dto.newBillingStartDate || defaultBillingStart,