ENG-6659: Fix uncentered 404 page (#1497)

Co-authored-by: Ahmad Hakim <[email protected]>

Author: carlosabadia

pcweb/pages/page404.py

@@ -11,10 +11,7 @@
 
 @webpage(path="/404", title="Page Not Found · Reflex.dev", add_as_page=False)
 def page404():
-    return rx.center(
-        rx.vstack(
-            markdown_with_shiki(contents),
-            rx.spacer(),
-        ),
-        class_name="h-[80vh] w-full",
+    return rx.box(
+        markdown_with_shiki(contents),
+        class_name="h-[80vh] w-full flex flex-col items-center justify-center",
     )