nebula-mesh: Unauthenticated OIDC login endpoint allocates unbounded in-memory state entries without rate limiting

Summary When OIDC is enabled, GET /ui/oidc/login is reachable without authentication and is registered outside the Web UI rate-limited auth routes. Every request creates a fresh random OIDC state value and stores it in an in-memory map for 10m. Expired states are swept lazily, but there is no rat...