Implement disabled localauth and idp name

* Use https://gotify.net/docs/config#gotify-oidc-idp-name as label for the login button.
* Hide login form when https://gotify.net/docs/config#gotify-localauth-enabled is disabled.

Start by tracing the Android login screen and how it reads the Gotify OIDC and local-auth configuration described in the linked documentation. Use those configuration values to determine the login button label and whether the local login form is shown. Done means both settings are reflected correctly in the login UI.

Written by the indexing model from the issue text.