feat: use Next.js app router + upgrade to next-auth@5 (beta)

[vincentchalamon]

https://auth-docs-git-feat-nextjs-auth-authjs.vercel.app/reference/nextjs https://auth-docs-git-feat-nextjs-auth-authjs.vercel.app/guides/upgrade-to-v5

• isomorphic-unfetch replaced by native fetch
• react/query replaced by @tanstack/react-query
• :warning: using next-auth@5-beta: https://github.com/nextauthjs/next-auth/pull/7443