Drupal sessions in angular app
I didn't use any session data, but I have passed sections of Drupal.settings on into the Angular scope when declaring modules. It worked for my case. Here is an example:

angular.module('mypackage.name', []).constant('appConfig', Drupal.settings.custom_module.custom_module_config);


This lived within the libraries folder (attached to pages by Drupal) for 7.x.

But I am not sure that it really matters what backend you have here, this seems like an Angular question. Here is one that might help: https://stackoverflow.com/questions/14957450/maintaining-session-through-angular-js