CalendarView constructor
Creates a CalendarView for the specified year and month.
Parameters:
year(int): Four-digit year valuemonth(int): Month number (1-12, where 1 = January)
Throws AssertionError if month is not between 1 and 12.
Example:
final view = CalendarView(2024, 3); // March 2024
Implementation
CalendarView(this.year, this.month) {
assert(month >= 1 && month <= 12, 'Month must be between 1 and 12');
}