getInitialMonthIndex method
Implementation
int getInitialMonthIndex() {
if (_selectedMonth == null) return 0;
final List<MonthRange> months = getAvailableMonths();
final int index = months.indexWhere(
(m) => m.startDate.isAtSameMomentAs(_selectedMonth!.startDate),
);
return index >= 0 ? index : 0;
}