showEventMenu method
Opens the entry menu (edit/delete) for entry at planner-local pos. The
caller (timed grid or all-day band) passes the chip's/event's
PlannerEntry, so the same menu serves both surfaces (#72).
Implementation
void showEventMenu(Offset pos, PlannerEntry<T> entry, Function onClose) {
menuPos = pos;
menuType = MenuType.entry;
menuEntry = entry;
menuTime = null;
_onCloseMenu = onClose;
}