Use 1 column layout for form `ul` on narrow widths (#32112)

app/javascript/styles/mastodon/forms.scss

@@ -312,6 +312,10 @@ code {
 
     ul {
       columns: 2;
+
+      @media screen and (max-width: $mobile-breakpoint) {
+        columns: 1;
+      }
     }
   }