I need my existing Next.js app wired up to an existing external REST API so it can securely fetch data, submit updates, handle authentication, and allow users to upload files. Most endpoints accept/return JSON, and uploads are sent as multipart/form-data — your code must handle both cleanly and reliably. Deliverables: A small, reusable, well-typed API client layer (e.g., /lib/api or similar) using fetch or Axios Authentication integration (bearer tokens / OAuth details provided). Requests should be signed correctly and handled consistently across the app Example usage: at least one working flow for: data retrieval data submission login/session handling file upload (multipart/form-data) Robust error handling (including loading + failure states) and a short README explaining patterns so I can extend them Important constraints: This is an integration task: do not redesign backend architecture or refactor the entire UI Keep changes focused and incremental, with clear commits If you’ve shipped Next.js projects integrating with REST backends and have handled both JSON + multipart/form-data securely, please share relevant examples. We’ll start with a small paid test task.