UX: Consistent padding on mobile & desktop `.wrap` (PR #13155)

Due to padding on .wrap, we were calculating with 16px on desktop, but mobile has slightly more padding so the 16px wasn’t enough. This caused the space on the right of the page to be 4px too small within topics on mobile.

The little extra padding on mobile is nice, but I don’t want to add more mobile-specific styles… so I gave desktop a little more padding instead and making everything consistent.