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/attendance/attendance-import.service.ts

@@ -2,6 +2,7 @@ import { BadRequestException, Injectable, Logger } from '@nestjs/common';
 import { InjectRepository } from '@nestjs/typeorm';
 import { Repository, In } from 'typeorm';
 import { Subject, Observable } from 'rxjs';
+import dayjs from '../common/dayjs';
 import {
   DingAttendanceRaw,
   Student,
@@ -238,7 +239,7 @@ export class AttendanceImportService {
   }
 
   private formatDate(value: Date): string {
-    return value.toISOString().slice(0, 10);
+    return dayjs.utc(value).format('YYYY-MM-DD');
   }
 
   /**