refactor: normalize date handling to Tehran timezone

- Updated date handling in BlogSeoFields and ScheduleSection to use Tehran timezone utilities for consistency.
- Introduced `toTehranClockTime`, `tehranWallClockToUnix`, and `todayIso` functions for accurate date representation.
- Modified various components to utilize these new utilities, ensuring that date strings are correctly formatted and timestamps are accurately converted.
- Enhanced API documentation to clarify the handling of date fields, emphasizing the importance of server-local midnight.
- Added tests to verify that date overrides and holidays maintain the correct day without shifting due to timezone discrepancies.

assets/admin/pages/ClaimsPage.tsx

@@ -3,7 +3,7 @@ import { useQuery } from '@tanstack/react-query';
 import { useNavigate, useSearchParams } from 'react-router-dom';
 import { ArrowPathIcon } from '@heroicons/react/24/outline';
 import { api } from '../lib/api';
-import { formatRial, formatNumber } from '../lib/utils';
+import { formatRial, formatNumber, toGregorianDate, todayIso } from '../lib/utils';
 import type { ClaimPatientRow } from '../types';
 import type { ApiResponse, PaginatedResponse } from '../lib/api';
 import PageHeader from '../components/ui/PageHeader';
@@ -35,9 +35,8 @@ const PAYMENT_OPTIONS = [
 const isoNDaysAgo = (days: number): string => {
   const d = new Date();
   d.setDate(d.getDate() - days);
-  return d.toISOString().slice(0, 10);
+  return toGregorianDate(d);
 };
-const todayIso = (): string => new Date().toISOString().slice(0, 10);
 
 const toUnix = (iso: string): number | null => {
   if (!iso) return null;