Fix login flow

[satyaog]

Fix login flow: async + single /token endpoint

• Make login() async and use await in Login.run() so the CLI correctly waits for the login result.
• Use a single /token?headless=true endpoint instead of /auth/login and /auth/login?headless=true.
• Simplify behavior: always call the token endpoint, then if headless or webbrowser.open() fails, print the login URL for the user; remove the “retry with headless” path.
• Return refresh_token from the token response instead of access_token.

[satyaog]

Depends on mila-iqia/easy-oauth#2