The browser treated sessionStorage as the source of truth for the logged-in
user and never verified or invalidated the server-side session cookie: no
GET /api/v1/demo/session or POST /api/v1/demo/logout endpoint existed, and a
central 401 handler was defined but never wired up.
Add both endpoints; the session-check response is marked Cache-Control:
no-store to avoid the browser serving a stale "authenticated" response right
after logout. AuthProvider now verifies against the server on every mount
(sessionStorage only caches presentation state to avoid a login-screen
flash), subscribes to a central 401 listener on the API client, and
RequireAuth shows a loading state during verification instead of flashing
protected content or the wrong role.