feat: add atproto-oauth-dioxus crate
Dioxus fullstack integration for AT Protocol OAuth authentication
with turnkey PKCE + DPoP flow. Provides:
- AtprotoOAuthProvider context component (session state + localStorage)
- AtprotoOAuthCallback component for the OAuth redirect
- do_atproto_login / do_atproto_logout hooks
- #[server] init_atproto_oauth / complete_atproto_oauth
- #[get] /oauth/client-metadata.json endpoint
- get_active_session() for downstream DPoP-authenticated PDS calls
- Feature-gated server deps (server, hickory-dns)