MM-59855 Fix layout issue on smaller screens when rhs is expanded (#28025)

* fixed layout isssue for <1200px when rhs is expanded

* border-radius on rhs when collapsed in 768-1200 range

webapp/channels/src/sass/base/_structure.scss

@@ -281,9 +281,10 @@ body.app__body #root {
                 width: 100%;
                 grid-area: center;
                 transition: width .25s ease-in-out;
+                border-radius: var(--radius-l);
             }
 
-            --columns: min-content auto min-content;
+            --columns: min-content 1fr min-content;
         }
 
         #sidebar-right {