Timer

The presenter page has a timer. Describe it in the headmatter:

---
duration: 30min
timer: countdown
---

• duration is 30min, 1h30min, 90s, or a number of minutes.
• timer is stopwatch (the default) or countdown.

The timer starts, pauses and resets from the presenter page, and the state is shared with the other presenter windows. A bar shows how much of the duration has passed.