chore: add 404 page

src/pages/404.astro

@@ -0,0 +1,24 @@
+---
+import Layout from '../layouts/Layout.astro'
+import '../styles/globals.css'
+import { siteConfigQuery } from '../graphql-documents/site-config.gql.generated'
+
+let siteTitle = ''
+let siteDescription = ''
+
+try {
+  const siteResult = await siteConfigQuery.execute()
+  siteTitle = siteResult.data?.siteConfig?.title ?? ''
+  siteDescription = siteResult.data?.siteConfig?.description ?? ''
+} catch {
+  // CMS may be unavailable on error pages
+}
+---
+
+<Layout title={siteTitle} description={siteDescription}>
+  <div class="min-h-screen grid content-center justify-items-center">
+    <div>404</div>
+    <div>Страница не найдена</div>
+    <a href="/" class="text-blue-600 hover:text-blue-700">На главную</a>
+  </div>
+</Layout>

src/pages/[slug].astro

@@ -54,7 +54,7 @@ if (categoryResult.errors?.length) {
 const category = categoryResult.data?.categories?.find(c => c != null)
 
 if (!category) {
-  throw new Error(`Category not found for slug: ${slug}`)
+  return new Response(null, { status: 404, statusText: 'Not Found' })
 }
 
 const title = category.title ?? undefined