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/post/[slug]/+page.server.ts

@@ -20,13 +20,22 @@ import { marked } from 'marked';
 
 marked.setOptions({ gfm: true });
 
-export const load: PageServerLoad = async ({ params, locals, url }) => {
+export const load: PageServerLoad = async ({ params, locals, 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 post = await getPostBySlug(slug, {
     locale: 'de',
     resolve: POST_RESOLVE,
+    preview,
   });
 
   if (!post) {
@@ -202,6 +211,7 @@ export const load: PageServerLoad = async ({ params, locals, url }) => {
 
   return {
     post,
+    preview: !!preview,
     postImageUrl,
     rawPostImageUrl: postImageField?.url ?? null,
     postImageFocal: postImageField?.focal ?? null,