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/bills/bills-generation.service.ts

@@ -4,6 +4,7 @@ import { DataSource, Repository } from 'typeorm';
 import { Bill, BillItem, RoomExpense, PersonalExpense, Occupancy, Room } from '../entities';
 import { WalletsService } from '../wallets/wallets.service';
 import type { GenerateBillsDto } from './dto/bill.dto';
+import dayjs from '../common/dayjs';
 
 @Injectable()
 export class BillsGenerationService {
@@ -235,7 +236,7 @@ export class BillsGenerationService {
       let year = startYear, month = startMonth;
       year < endYear || (year === endYear && month <= endMonth);
     ) {
-      const daysInMonth = new Date(Date.UTC(year, month, 0)).getUTCDate();
+      const daysInMonth = dayjs.utc(`${year}-${month}`).daysInMonth();
       const prefix = `${year}-${String(month).padStart(2, '0')}-`;
       const overlapStart = activeStart > `${prefix}01` ? activeStart : `${prefix}01`;
       const monthEnd = `${prefix}${String(daysInMonth).padStart(2, '0')}`;
@@ -258,7 +259,7 @@ export class BillsGenerationService {
   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;
   }