fix: use position fixed for body::before content shadow

Prevents the shadow column from cutting off at the fold when body has
height: 100%. Fixed positioning makes the pseudo-element always cover
the full viewport regardless of body sizing.

Author: Leftium

src/_grid-columns.scss

@@ -26,15 +26,15 @@
     min-width: 0;
   }
 
-  // Content shadow: absolutely-positioned pseudo spanning full body height.
+  // Content shadow: fixed-position pseudo spanning the full viewport height.
   // Width = content column + shadow gap on each side, creating a visual
   // separation between the shadow edge and the content.
   // The clamp() formula handles visibility: shadow shows at full width when
   // there's at least 2*gap of space on each side between shadow edge and
   // viewport edge, otherwise snaps to 0. No breakpoint needed.
   body::before {
     content: '';
-    position: absolute;
+    position: fixed;
     top: 0;
     bottom: 0;
     left: 50%;