fix error page layout stacking blocks side by side

The error-main flex container was missing flex-direction: column,
which went unnoticed until the page builder added a second block
(featured_products) to the error page defaults.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

assets/css/shop/components.css

@@ -2392,6 +2392,7 @@
 
   .error-main {
     display: flex;
+    flex-direction: column;
     align-items: center;
     justify-content: center;
     min-height: calc(100vh - 4rem);