Implement user login

[DrStrangeloovee]

Continuing from the design (#20) the login functionality will be implemented.

The underlying issue (#29) needs to be implemented first to utilize a uniform way of accessing the auth API which pocketbase provides.

Concrete implementation:

1. User is able to login by providing a username/email and password
2. User successfully authenticates and will receive a session in the form of a cookie which is passed on for the subsequent requests to the API
3. Ensure there is an easy way to check for an authenticated user in the front end

Afterwards we should create a simple page/route which is only accessible for the authenticated user otherwise the user should be redirected to an error page or something similar.