Option to hide unused sign-in methods

Thanks for adding generic OIDC in v0.33.8. It works great with Authentik on my self-hosted instance.

At the moment the login page always shows Google, GitHub, Apple and passkey buttons, even when no client IDs are configured. On a self-hosted setup that uses OIDC only, those buttons do nothing and confuse users.

Suggestion: an env var such as AUTH_HIDE_SOCIAL_PROVIDERS=true (or a list, e.g. AUTH_ENABLED_PROVIDERS=oidc,passkey,email). The backend could expose it through the existing /auth/features endpoint, and login.vue could render each button conditionally, the same way the OIDC button already does. Alternatively, buttons could be shown only when their *_CLIENT_ID is set.

Thanks!