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/AppointmentsPage.tsx

@@ -9,7 +9,7 @@ import { toast } from 'sonner';
 import { api } from '../lib/api';
 import type { PaginatedResponse, ApiResponse } from '../lib/api';
 import type { Appointment } from '../types';
-import { formatDate, toGregorianDate, formatTime, digitsOnly, sanitizeMobileInput, rialToToman, tomanToRial, formatRial } from '../lib/utils';
+import { formatDate, toGregorianDate, todayIso, formatTime, digitsOnly, sanitizeMobileInput, rialToToman, tomanToRial, formatRial } from '../lib/utils';
 import PriceInput from '../components/ui/PriceInput';
 import Modal from '../components/ui/Modal';
 import { useAuthStore } from '../stores/authStore';
@@ -507,7 +507,7 @@ export default function AppointmentsPage() {
   const canCancelAppt = can('appointments', 'cancel');
 
   const [params] = useSearchParams();
-  const today = new Date().toISOString().slice(0, 10);
+  const today = todayIso();
   // پس از ویرایش/ثبت، صفحه با ?date=... باز می‌شود تا همان روز نمایش داده شود.
   const [selectedDate, setSelectedDate] = useState(params.get('date') || today);
   const [viewMode, setViewMode] = useState<TurnsViewMode>('timeline');