Fix RSC build break: keep next/headers out of lib/payload.ts

payload.ts's mapping functions/types are also imported by "use client"
components (LiveTestimonialsGrid, LivePostContent) — importing
next/headers anywhere in that module made it unbundlable for the client,
breaking the production build. draftMode() is now only ever called in the
Server Component pages themselves; they pass the resulting boolean into
getPostBySlug/getLegalPage/getTestimonials as a plain `draft` option.

app/agb/page.tsx

@@ -17,9 +17,9 @@ export const metadata: Metadata = {
 };
 
 export default async function AgbPage() {
-  const page = await getLegalPage("agb");
-  const headings = page ? extractHeadings(page.content) : [];
   const { isEnabled: isPreview } = await draftMode();
+  const page = await getLegalPage("agb", { draft: isPreview });
+  const headings = page ? extractHeadings(page.content) : [];
 
   return (
     <>