feat(preview): implement preview mode for content management

- Added support for a preview mode in the CMS, allowing draft content to be viewed without being indexed.
- Updated hooks and server logic to handle preview parameters, adjusting cache control and robots tags accordingly.
- Enhanced layout to display a preview notification when in preview mode, improving user experience for content editors.

src/routes/[...slug]/+page.server.ts

@@ -23,13 +23,22 @@ function focalToCss(focal: { x: number; y: number } | undefined): string | null
   return `${(focal.x * 100).toFixed(2)}% ${(focal.y * 100).toFixed(2)}%`;
 }
 
-export const load: PageServerLoad = async ({ params, locals, fetch }) => {
+export const load: PageServerLoad = async ({ params, locals, fetch, url, setHeaders }) => {
   const { slug } = params;
   const translations = locals.translations ?? {};
+  const preview = url.searchParams.get('preview') ?? undefined;
+
+  if (preview) {
+    setHeaders({
+      'cache-control': 'private, no-store, max-age=0',
+      'x-robots-tag': 'noindex, nofollow',
+    });
+  }
 
   const page = await getPageBySlug(slug, {
     locale: 'de',
     resolve: PAGE_RESOLVE,
+    preview,
   });
 
   if (!page) {
@@ -78,6 +87,7 @@ export const load: PageServerLoad = async ({ params, locals, fetch }) => {
   return {
     page,
     translations,
+    preview: !!preview,
     seoTitle: page.seoTitle ?? page.headline ?? page.linkName ?? slug,
     seoDescription: page.seoDescription ?? page.subheadline ?? '',
     robots: (page as { seoMetaRobots?: string }).seoMetaRobots ?? undefined,