fix: Demo navigation via history API and route sync.

Replace unreliable pathname assignment with pushState and central route syncing so the demo opens from the login screen and responds to browser back/forward.

Co-authored-by: Cursor <cursoragent@cursor.com>

client/src/components/AuthOnboarding.tsx

@@ -16,9 +16,10 @@ import RegistrationDisclaimer from './RegistrationDisclaimer.tsx'
 
 interface AuthOnboardingProps {
   onAuthenticated: () => void
+  onOpenDemo?: () => void
 }
 
-export default function AuthOnboarding({ onAuthenticated }: AuthOnboardingProps) {
+export default function AuthOnboarding({ onAuthenticated, onOpenDemo }: AuthOnboardingProps) {
   const { t, i18n } = useTranslation()
   const [username, setUsername] = useState('')
   const [loading, setLoading] = useState(false)
@@ -526,7 +527,7 @@ export default function AuthOnboarding({ onAuthenticated }: AuthOnboardingProps)
         <button
           type="button"
           className="btn secondary"
-          onClick={() => { window.location.pathname = '/demo' }}
+          onClick={() => onOpenDemo?.()}
           disabled={loading}
           style={{ width: '100%' }}
         >