Set specific width for sidebar column in CSS grid

Having this set to "auto" may be contributing to the layout shift while
the page is loading, since it doesn't know the size until the content is
loaded.

tildes/scss/_layout.scss

@@ -6,7 +6,7 @@ body {
     display: grid;
     grid-template-rows: auto 1fr auto;
 
-    grid-template-columns: 1fr minmax(auto, $main-max-width) auto 1fr;
+    grid-template-columns: 1fr minmax(auto, $main-max-width) $sidebar-width 1fr;
     grid-template-areas:
       ". header header  ."
       ". main   sidebar ."