Angular is a thick client, served by Node.js, primarily interfacing with a separate RESTful web service.
Node.js comes in for user authentication, communicating with an oAuth provider, getting the returned data, and looking up or creating the user on the web service. It then stores that user data as a session in Mongo. The user data is then made available to angular as JSON from Node.js.
Once authenticated, the Angular app should communicate directly with the RESTful web service.