fix: skip stale token validation during callback code exchange (refs #190)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/core/auth/Auth0Provider.tsx

@@ -120,6 +120,7 @@ const TokenInjector: React.FC<{ children: React.ReactNode }> = ({ children }) =>
   // Prevent stale session state when cached token is no longer valid
   React.useEffect(() => {
     if (!isAuthenticated || isLoading || validatingRef.current) return;
+    if (window.location.pathname === '/callback') return;
 
     const validateToken = async () => {
       validatingRef.current = true;