DateTime.UnixTimestamp now handles MinValue and MaxValue across timezones

Calling DateTime.UnixTimestamp on a sentinel value (DateTime.MinValue, returned by an unset DateTimePicker.DisplayDateStart, or a near-DateTime.MaxValue from DisplayDateEnd) crashed the app with an ArgumentOutOfRangeException in timezones whose offset pushes the converted UTC instant outside the representable year 1..9999 range (e.g. Europe/Paris, America/New_York). UnixTimestamp now clamps the converted instant to the representable range and returns a finite value instead of crashing.