Authentication and authorization with existing web app

The starting point for my scenario is a node/express/angular/postgres (pean.js) website already exists and uses passport.js local strategy. Both $http and calls are made to retrieve data, with $http alone being used for user login.

If I made an ionic 2 app, what would my options be for authentication and authorization?

There seems to be an assumption that when making an ionic 2 app that you are always starting from a blank slate in terms of who your users are and what their application roles are. The ionic blog posts related to authorization are not helpful for this case.

If you truly are building a Progressive Web App, I suppose you do not have this problem, because you really are building only one app - starting on the web - and therefore only have one app to maintain.