x/tools/present: fix rendering on mobile

[mvdan]

Trying to follow a presentation on a mobile device is currently very difficult. Some of the issues include:

• Scrolling left or right sometimes doesn't move one slide; rather, it scrolls through the visible horizontal slides
• The screens tend to be tall, meaning that the text is too small by default.
• Trying to zoom in our out easily breaks the UI; one can end up with slides that are cut off.

Below is a screenshot I took after zooming in and out on a presentation of mine.

My HTML/JS skills are limited, so I don't know if this would be a major rework for mobile, or just some tweaking to have it behave better on small/tall screens.