What is Next.js and what problems does it solve?

// app/page.tsx — runs on the server, SEO-friendly HTML, zero client JS
export default async function Home() {
  const posts = await fetch('https://api/posts').then(r => r.json());
  return (
    <ul>
      {posts.map((p) => <li key={p.id}>{p.title}</li>)}
    </ul>
  );
}