fix: prevent logout loop by showing signed-out state instead of auto-redirecting

After logout, redirect to /platform/login?signed_out which shows a
"Signed out" card with a "Sign in again" button instead of immediately
redirecting back to Logto OIDC (which would auto-authenticate if the
Logto session cookie persists).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/src/auth/useAuth.ts

@@ -7,7 +7,7 @@ export function useAuth() {
   const { currentTenantId } = useOrgStore();
 
   const logout = useCallback(() => {
-    signOut(window.location.origin + '/platform/login');
+    signOut(window.location.origin + '/platform/login?signed_out');
   }, [signOut]);
 
   return {