Show remaining time for open tasks in Time Slot hover

Time Slot: show remaining time (exclude completed tasks)

When hovering a Time Slot, the total duration counts all tasks, including completed ones. Example: the slot is 1h 45m, the hover shows 3h 3m – but 5 of 14 tasks are already done and the open ones only need 1h 47m. Show the remaining time of open tasks (e.g. “1h 47m left / 3h 3m total”), so it is visible at a glance whether the slot is still realistic.