Fancy oauth login

This would probably require work on the api side too, but it would be great if, on the CLI that ships with this npm now, you could do something like transloadit auth login, and a browser opens, a code is entered, CLI specific auth keys are created/downloaded to .env.

Example: Convex has:

npm create convex@latest

Potentially we can have something similar.

TODO (auth + onboarding)

• Define API device-login flow: browser opens, user logs in, enters code, CLI receives scoped keys.
• Implement transloadit auth login (device code flow) and save keys to .env and ~/.transloadit.
• Ship npm create transloadit scaffold that:
  1) opens browser login,
  2) waits for code entry,
  3) writes .env,
  4) validates a demo Assembly.
• Add CLI E2E harness (Playwright or similar) to continuously verify the login/scaffold flow.
• Add a “no browser” fallback for CI (token paste or TRANSLOADIT_KEY/SECRET env).

The issue names no files or tests. Start by locating the existing CLI entry point and API authentication code, then break the TODO into device login, credential storage, npm create scaffolding, E2E coverage, and the CI fallback. Done means the API flow and each listed CLI/onboarding path are implemented and the login/scaffold flow is continuously verified.