UX: remove horizontal scoll from narrow screens (#16657)

app/assets/stylesheets/common/base/header.scss

@@ -14,6 +14,7 @@
   backface-visibility: hidden; /** do magic for scrolling performance **/
 
   > .wrap {
+    box-sizing: border-box;
     width: 100%;
     height: 100%;
 

app/assets/stylesheets/desktop/discourse.scss

@@ -184,6 +184,7 @@ input {
 }
 
 #main-outlet-wrapper {
+  box-sizing: border-box;
   width: 100%;
   display: grid;
   grid-template-areas: "content";