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/todo-cards/page.tsx

@@ -33,8 +33,8 @@ export const metadata: Metadata = {
 };
 
 export default async function TodoCardsPage() {
-  const testimonials = await getTestimonials("todo-cards");
   const { isEnabled: isPreview } = await draftMode();
+  const testimonials = await getTestimonials("todo-cards", { draft: isPreview });
 
   return (
     <>