Add /auth/login and /auth/callback handlers. The backend holds the
client secret and exchanges the authorization code server-side;
the browser never sees the secret. Redirect URI and post-login
fragment destination are derived solely from GATEWAY_SERVER_PUBLIC_URL
(required when OIDC is set) to prevent host-header injection.
Tailwind v4 dark design (DM Sans/Mono, violet accent). Pages: Overview,
Logs, API Keys, Providers, Models, Analytics, Routing, Settings.
Auth gate uses localStorage token validated against /api/admin/providers.
Vite builds to internal/web/dist which is embedded into the binary.