web: disable pull-to-refresh on mobile

[fuerchter]

Hi, I added a line in style.css to disable pull-to-refresh on mobile, see this and this for some documentation of the feature.

Videos demonstrating it on Android Chrome:

https://github.com/fogleman/rush/assets/3199489/03a94f15-4554-4142-bfbe-ade6b1568f68

https://github.com/fogleman/rush/assets/3199489/10a3e4b6-5508-47af-abaf-8afe1583194e