Fix corner cases in the Agenda and Month views
- If an incidence starts and also ends or is due at 00:00 of a day, the month view displayed it as a thin rectangle on the border between days.
- Events that begin and end at 00:00 of the same day were displayed in the agenda across the whole day, as if they had 24-hour duration.
- One-day events starting after 23:52:30 and ending at or before midnight were displayed below the bottom of the agenda.