Implement auth component for Vue

The [`thin-backend-react/auth-ui.tsx`](https://github.com/digitallyinduced/thin-backend/blob/master/thin-backend-react/auth-ui.tsx) module should be ported to Vue to `thin-backend-vue`, so that we can provide in-app login in Vue apps as well 👍

Start by reading thin-backend-react/auth-ui.tsx to understand the existing authentication UI and identify the corresponding thin-backend-vue entry point. Port the component's behavior to Vue so Vue applications provide equivalent in-app login functionality; verify the new module builds and can be used from a Vue app.

Written by the indexing model from the issue text.