Update unified starters (Flow+Hilla) to use stateful authentication

[mshabarov]

Describe your motivation

Any unified Vaadin starter that uses authentication should use stateful authentication so that it can work with both Hilla views and Flow views. Starters that previously didn't use authentication will remain without authentication.

Additional context

Vaadin 24.4.

[mshabarov]

Here is a draft branch that has hybrid app with stateful auth - https://github.com/vaadin/flow-hilla-hybrid-example/tree/stateful-auth.