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/lib/cms.ts

@@ -229,6 +229,8 @@ export type ContentGetOptions = {
   locale?: string;
   /** Felder, deren Referenzen aufgelöst werden (z. B. ["row1Content", "row2Content", "row3Content"]). */
   resolve?: string[];
+  /** Signierter Preview-Token (HMAC), entsperrt Drafts für genau diesen Eintrag. Bypass Cache. */
+  preview?: string;
 };
 
 /** Führenden + abschließenden Slash entfernen (CMS "slug": "/about"; Route-Param mit trailing slash). */
@@ -245,13 +247,14 @@ export async function getPageBySlug(
 ): Promise<PageEntry | null> {
   const resolveKey = options?.resolve?.join(",") ?? "";
   const key = `page:slug:${slug}:${options?.locale ?? ""}:${resolveKey}`;
-  return cached("page", key, async () => {
+  const fetchFn = async () => {
     const base = getBaseUrl();
     const trySlug = async (s: string): Promise<Response> => {
       const u = new URL(`${base}/api/content/page/${encodeURIComponent(s)}`);
       if (options?.locale) u.searchParams.set("_locale", options.locale);
       if (options?.resolve?.length)
         u.searchParams.set("_resolve", options.resolve.join(","));
+      if (options?.preview) u.searchParams.set("preview", options.preview);
       return fetch(u.toString());
     };
     const normSlug = normalizePageSlug(slug);
@@ -277,7 +280,9 @@ export async function getPageBySlug(
       );
     }
     return (await res.json()) as PageEntry;
-  });
+  };
+  if (options?.preview) return fetchFn();
+  return cached("page", key, fetchFn);
 }
 
 /** Für Page-URLs wird `slug` bevorzugt (nicht _slug). Führender Slash wird ignoriert. */
@@ -435,13 +440,14 @@ export async function getPostBySlug(
 ): Promise<PostEntry | null> {
   const resolveKey = options?.resolve?.join(",") ?? "";
   const key = `post:slug:${slug}:${options?.locale ?? ""}:${resolveKey}`;
-  return cached("post", key, async () => {
+  const fetchFn = async () => {
     const base = getBaseUrl();
     const trySlug = async (s: string): Promise<Response> => {
       const u = new URL(`${base}/api/content/post/${encodeURIComponent(s)}`);
       if (options?.locale) u.searchParams.set("_locale", options.locale);
       if (options?.resolve?.length)
         u.searchParams.set("_resolve", options.resolve.join(","));
+      if (options?.preview) u.searchParams.set("preview", options.preview);
       return fetch(u.toString());
     };
     const normSlug = normalizePostSlug(slug);
@@ -467,7 +473,9 @@ export async function getPostBySlug(
       );
     }
     return (await res.json()) as PostEntry;
-  });
+  };
+  if (options?.preview) return fetchFn();
+  return cached("post", key, fetchFn);
 }
 
 /** Für Post-URLs wird slug bevorzugt (nicht _slug). Führender Slash wird ignoriert. */