MonthRangePicker class

A custom date picker modal that allows users to select a month/year or a date range.

This widget provides two modes:

• Monthly: Select a specific month and year
• Custom: Select a custom date range with preset options (Last 3 months, Last 6 months, or custom range)

Example usage:

final result = await CustomDatePickerModal.show(
  context,
  initialDate: DateTime.now(),
  firstDate: DateTime(2020),
  lastDate: DateTime.now(),
);

if (result != null) {
  final (selectedDate, dateTimeRange, actionType) = result;
  if (actionType == ActionType.confirm) {
    // Handle date selection
  }
}