fix(appointments): stay on the same day after edit/create

AppointmentsPage now reads an optional ?date=YYYY-MM-DD query param as its
initial selected date. AppointmentEditPage and AppointmentCreatePage navigate
back to /admin/appointments?date=<the appointment's day> on save (and the edit
page's back link carries the date too), so the user returns to the day they
were viewing instead of today.

Test: AppointmentsPage honors ?date= and fetches that day.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

assets/admin/pages/AppointmentCreatePage.tsx

@@ -116,7 +116,8 @@ export default function AppointmentCreatePage() {
     onSuccess: () => {
       qc.invalidateQueries({ queryKey: ['appointments'] });
       toast.success('نوبت با موفقیت ثبت شد');
-      navigate('/admin/appointments');
+      // به همان روزِ نوبت برگرد، نه امروز.
+      navigate(`/admin/appointments?date=${date}`);
     },
     onError: (e: any) => toast.error(e.message || 'خطا در ثبت نوبت'),
   });