fix: restore original URL after OAuth login redirect

Save the intended destination to sessionStorage before navigating to
the OAuth provider, and read it back in AuthCallback after login.
Also handles 401-triggered redirects so session-expired users are
returned to the page they were on.

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

frontend/src/pages/AuthCallback.tsx

@@ -11,7 +11,11 @@ function AuthCallback() {
   useEffect(() => {
     const handleCallback = async () => {
       const token = searchParams.get('token');
-      const redirect = searchParams.get('redirect') || '/';
+      const redirect =
+        searchParams.get('redirect') ||
+        sessionStorage.getItem('oauth_redirect') ||
+        '/';
+      sessionStorage.removeItem('oauth_redirect');
 
       if (!token) {
         setError('No authentication token received');