fix(layout): remove mobile body scroll to keep one scroll container

Both html/body and #main-app being scrollable on mobile made
scrollIntoView animate two ancestors at once, shifting the start
frame of the smooth scroll. With body locked, #main-app is the only
scroller and the animation matches the user's actual position.

src/styles/app.css

@@ -62,13 +62,6 @@ body {
   scroll-behavior: smooth;
 }
 
-@media screen and (max-width: 768px) {
-  html,
-  body {
-    overflow-y: auto;
-  }
-}
-
 .columns {
   margin-left: 0;
   margin-right: 0;