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/blog/[slug]/page.tsx

@@ -39,7 +39,8 @@ export default async function BlogDetailPage({
   params: Promise<{ slug: string }>;
 }) {
   const { slug } = await params;
-  const post = await getPostBySlug(slug);
+  const { isEnabled: isPreview } = await draftMode();
+  const post = await getPostBySlug(slug, { draft: isPreview });
   if (!post) notFound();
 
   // "Weiterlesen" — any other post, most recent first. Not the current
@@ -47,7 +48,6 @@ export default async function BlogDetailPage({
   // showing a fake/duplicate card when this is the only post.
   const otherPosts = await getBlogPosts(4);
   const nextPost = otherPosts.find((p) => p.slug !== post.slug) ?? null;
-  const { isEnabled: isPreview } = await draftMode();
 
   return (
     <>