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/classes/classes-queries.service.ts

@@ -5,6 +5,7 @@ import { Class, ClassStudent, ClassSchedule, AttendanceRecord } from '../entitie
 import { Classroom } from '../entities/classroom.entity';
 import { syncDingTalkStudents } from '../integration/dingtalk-student-sync';
 import type { QueryClassScheduleDto, QueryClassAttendanceSummaryDto } from './dto/class.dto';
+import dayjs from '../common/dayjs';
 
 interface AgentClassRow {
   id: string | number;
@@ -80,7 +81,7 @@ export class ClassesQueriesService {
     const existingByStudentId = new Map(
       existingClassStudents.map((classStudent) => [classStudent.studentId, classStudent]),
     );
-    const today = new Date().toISOString().slice(0, 10);
+    const today = dayjs().utc().format('YYYY-MM-DD');
     let skipped = 0;
     const memberships = studentIds.flatMap((studentId) => {
       const existing = existingByStudentId.get(studentId);