Refactor API configuration and improve WebSocket handling in frontend and backend

frontend/src/pages/LandingPage.tsx

@@ -1,9 +1,8 @@
 import FloatingGem from '../components/PixelSprites/FloatingGem';
 import PixelIcon from '../components/PixelIcon/PixelIcon';
+import { API_BASE_URL } from '../config';
 import './LandingPage.css';
 
-const API_BASE_URL = import.meta.env.VITE_API_URL || "https://docnest-backend-mingda.fly.dev/api";
-
 function LandingPage() {
   const handleGoogleLogin = () => {
     window.location.href = `${API_BASE_URL}/auth/google`;