Fix invalid month overflow in totalDaysInMonth

[Copilot]

DateComponents with month: 13 silently overflows to January of the following year instead of returning nil, causing totalDaysInMonth to return 31 for invalid months rather than the expected 0.

Changes

• Date.startOfMonth(for:of:using:) — added an early guard (1...12).contains(month) check that returns nil for out-of-range months before constructing DateComponents, preventing the calendar overflow

static func startOfMonth(for month: Int, of year: Int, using calendar: Calendar = .current) -> Date? {
    guard (1...12).contains(month) else { return nil }
    return DateComponents(calendar: calendar, year: year, month: month).date
}

This nil propagates through the existing guard in totalDaysInMonth, making it return 0 as intended.