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/BlogSeoFields.tsx

@@ -2,6 +2,7 @@ import { Controller, useFieldArray } from 'react-hook-form';
 import type { Control, UseFormRegister } from 'react-hook-form';
 import type { BlogFormData } from '../lib/blogForm';
 import PersianDatePicker from './ui/PersianDatePicker';
+import { toGregorianDate, toTehranClockTime, tehranWallClockToUnix } from '../lib/utils';
 
 interface Props {
     control: Control<BlogFormData>;
@@ -90,16 +91,16 @@ export default function BlogSeoFields({ control, register }: Props) {
     );
 }
 
-/** Gregorian date + time → unix seconds, and back. */
+/** Gregorian date + time → unix seconds, and back — همیشه به وقت ایران. */
 function ScheduleField({ value, onChange }: { value: number | null; onChange: (v: number | null) => void }) {
     const d = value ? new Date(value * 1000) : null;
-    const dateStr = d ? d.toISOString().slice(0, 10) : '';
-    const timeStr = d ? d.toTimeString().slice(0, 5) : '00:00';
+    const dateStr = d ? toGregorianDate(d) : '';
+    const timeStr = d ? toTehranClockTime(d) : '00:00';
 
     const emit = (nextDate: string, nextTime: string) => {
         if (!nextDate) return onChange(null);
-        const ms = Date.parse(`${nextDate}T${nextTime || '00:00'}:00`);
-        onChange(Number.isNaN(ms) ? null : Math.floor(ms / 1000));
+        const ts = tehranWallClockToUnix(nextDate, nextTime || '00:00');
+        onChange(Number.isNaN(ts) ? null : ts);
     };
 
     return (