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/newsletter/page.tsx

@@ -39,8 +39,8 @@ export const metadata: Metadata = {
 // weekly newsletter), so this page resolves both at once instead of
 // picking a new URL and leaving one of them still dangling.
 export default async function NewsletterPage() {
-  const testimonials = await getTestimonials("newsletter");
   const { isEnabled: isPreview } = await draftMode();
+  const testimonials = await getTestimonials("newsletter", { draft: isPreview });
 
   return (
     <>