Feature Idea : add SSO authentification

[PloxPlox]

Feature Request: SSO / OIDC Authentication Support

Problem

Fladder authenticates by calling the Jellyfin API directly (/Users/AuthenticateByName), which bypasses any SSO layer. Users who protect their Jellyfin instance behind a Forward Auth middleware (Traefik, Caddy, nginx...) combined with an identity provider (Authentik, Authelia, Keycloak, etc.) either can't use Fladder at all, or have to whitelist raw API endpoints and maintain separate credentials.

Proposed Solution

Add an optional OIDC/OAuth2 login flow that redirects to the identity provider (via the jellyfin-plugin-sso endpoints), then exchanges the returned token for a native Jellyfin session — similar to how many mobile apps handle SSO by opening an external browser.

This would let Fladder fit naturally into hardened self-hosted setups without compromising security.