fix: use dynamic URLs for API and Socket.IO

- Change API_URL from hardcoded 'http://localhost:8080/api' to '/api'
- Change SOCKET_URL from hardcoded to window.location.origin
- Fixes production build connecting to wrong port
- Now works correctly in both dev (localhost:8080) and prod (localhost)

frontend/src/services/api.js

@@ -1,4 +1,6 @@
-const API_URL = 'http://localhost:8080/api';
+// Use relative URL - works for both dev (localhost:8080) and prod (localhost)
+// Nginx proxies /api to backend
+const API_URL = '/api';
 
 class ApiError extends Error {
   constructor(message, status, data) {