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/PatientRecordFormPage.tsx

@@ -12,7 +12,7 @@ import type { PatientRecord } from '../types';
 import PersianDateInput from '../components/ui/PersianDateInput';
 import SearchableSelect from '../components/ui/SearchableSelect';
 import { numericField } from '../lib/forms';
-import { iranNationalCodeSchema, iranMobileSchema } from '../lib/utils';
+import { iranNationalCodeSchema, iranMobileSchema, unixToIso } from '../lib/utils';
 
 const REFERRAL_OPTIONS = ['اینستاگرام', 'معرفی دوستان و آشنایان', 'جستجوی اینترنتی', 'تابلو مطب', 'سایر'];
 
@@ -29,7 +29,7 @@ const schema = z.object({
 type Form = z.infer<typeof schema>;
 
 const toEpoch = (iso?: string) => (iso ? Math.floor(new Date(iso).getTime() / 1000) : null);
-const fromEpoch = (ts?: number | null) => (ts ? new Date(ts * 1000).toISOString().slice(0, 10) : '');
+const fromEpoch = (ts?: number | null) => unixToIso(ts);
 
 /** تشکیل/ویرایش پرونده — patient record create & edit form (Figma "تشکیل پرونده"). */
 export default function PatientRecordFormPage() {