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/expenses/expense-operations.service.ts

@@ -5,6 +5,7 @@ import { DataSource, In, Repository } from 'typeorm';
 import { RoomExpense, PersonalExpense, Room, Student } from '../entities';
 import { BillsService } from '../bills/bills.service';
 import { RoomsService } from '../rooms/rooms.service';
+import dayjs from '../common/dayjs';
 import type { CreatePersonalExpenseDto } from './dto/expense.dto';
 
 @Injectable()
@@ -28,7 +29,7 @@ export class ExpenseOperationsService {
   private isValidDate(value: string) {
     if (!/^\d{4}-\d{2}-\d{2}$/.test(value || '')) return false;
     const date = new Date(`${value}T00:00:00Z`);
-    return !Number.isNaN(date.getTime()) && date.toISOString().slice(0, 10) === value;
+    return !Number.isNaN(date.getTime()) && dayjs(date).utc().format('YYYY-MM-DD') === value;
   }
 
   async createPersonalExpense(dto: CreatePersonalExpenseDto, userId?: number) {