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/components/ui/PersianCalendar.tsx

@@ -1,5 +1,6 @@
 import React, { useEffect, useRef, useState } from 'react';
 import { ChevronRightIcon, ChevronLeftIcon } from '@heroicons/react/24/outline';
+import { todayIso } from '../../lib/utils';
 
 interface Props {
   value: string;        // YYYY-MM-DD Gregorian
@@ -70,7 +71,8 @@ const JALALI_MONTHS = ['فروردین','اردیبهشت','خرداد','تیر'
 export default function PersianCalendar({ value, onChange, onClose, enableYearPicker = false }: Props) {
   const ref = useRef<HTMLDivElement>(null);
 
-  const todayGreg = new Date().toISOString().slice(0, 10);
+  // «امروز» به وقت ایران، نه UTC: `toISOString` بعد از ۲۰:۳۰ فردا را امروز نشان می‌داد.
+  const todayGreg = todayIso();
   const todayJ = toJalali(new Date(todayGreg + 'T12:00:00'));
   const valueJ = value ? toJalali(new Date(value + 'T12:00:00')) : todayJ;