The SSO button on the login page was not appearing because the settings
API requires authentication, but the login page cannot authenticate before
the user logs in.
Changes:
- Backend: Add GET /api/v1/auth/sso/config public endpoint that returns
only enabled, display_name, and auth_url (no secrets exposed)
- Backend: Mount sso::public_router() at /api/v1/auth/sso in main.rs
(was previously missing - only azure_compat_router was mounted)
- Frontend: Replace settingsApi.get() call in LoginPage.tsx with
ssoConfigApi.get() which calls the public endpoint
- Frontend: Add SsoConfigResponse interface and ssoConfigApi helper
to client.ts
- Frontend: Use auth_url from config response instead of hardcoded path