Fix calendar prompt for full day event dates

[micz]

The calendar event for full date events must have the endDate set to the day after, otherwise Thunderbird will show the day before.

[micz]

This fix will be released in version 4.1.0.
Anyone can add this statement in the current prompt to fix the problem in the current version:

If the event is a full day event, endDate must be one day after startDate with time set to "T000000"

[github-actions]

This feature is available for testing in the pre-release version v4.1.0pre3.