Availability windows

Controls a per-cell choice on the schedule grid. When ON, each exam cell gets a window mode; when OFF the control is hidden and every entry behaves as a fixed window.

• Fixed window — the exam runs [start_time → start_time + duration]. A late joiner loses the elapsed time; the deadline is the window end.
• Full duration — the student gets the entire duration from whenever they start inside the availability window (deadline = start + duration).

Choose fixed for a strict sitting, full-duration for flexible arrival.