fix: prevent duplicate navbar on 404 page (#9862)

Override the page-header slot in 404.astro to explicitly render
Navigation once, preventing duplication when Astro.rewrite('/404')
is called from dynamic route pages.

Fixes #9841

Co-authored-by: Your Name <your@email.com>

Author: adinjithj

src/pages/404.astro

@@ -1,6 +1,7 @@
 ---
 import Icon from '../components/AstroIcon.astro';
 import BaseLayout from '../layouts/BaseLayout.astro';
+import Navigation from '../components/Navigation/Navigation.astro';
 import { listOfficialRoadmaps } from '../queries/official-roadmap';
 
 const roadmapIds = await listOfficialRoadmaps();
@@ -20,6 +21,8 @@ const legacyRoadmapUrls = [
     }
   </script>
 
+  <Navigation slot='page-header' />
+
   <div class='bg-gray-100'>
     <div
       class='container flex flex-col items-center justify-center gap-7 py-10 md:flex-row md:py-32'