Create a package for user authentication

I've copied the approach used by i15-1 to handle user authentication (UserAuthContext, etc.) in the p99 app. It would be much better if there is a package for this that all apps can use.

Start by comparing the user-authentication approach used by i15-1 with UserAuthContext and related code in the p99 app. Determine the shared package's scope and how all apps would consume it; done means the authentication behavior is reusable across the apps without duplicating the existing approach.

Written by the indexing model from the issue text.