Make PDF zooming more responsive by using a lower throttle wait

Closes #364 (closed)

PDF zooming currently has a 500 millisecond debounce, which doesn't feel smooth. But we do need some kind of throttling since the zoom value modifies local storage and re-renders the whole page (= it's costly).

To counter this, I implement two changes:

• Change from debouncing to throttling (single changes are immediately reflected -- feels smoother)
• Reduce throttle wait time from 500 to 50ms. This is still miles better than no threshold at all, and it feels acceptably responsive to user input.