fix: Add production fallbacks for API and WebSocket URLs

frontend/src/api/client.ts

@@ -1,4 +1,4 @@
-const API_BASE_URL = import.meta.env.VITE_API_URL || "http://localhost:8080/api";
+const API_BASE_URL = import.meta.env.VITE_API_URL || "https://docnest-backend-mingda.fly.dev";
 
 export async function authFetch(url: string, options?: RequestInit): Promise<Response> {
   const token = localStorage.getItem('auth_token');

frontend/src/lib/yjs.ts

@@ -4,7 +4,7 @@ import { WebsocketProvider } from "y-websocket";
 import * as Y from "yjs";
 import { documentsApi } from "../api/document";
 
-const WS_URL = import.meta.env.VITE_WS_URL;
+const WS_URL = import.meta.env.VITE_WS_URL || "wss://docnest-backend-mingda.fly.dev";
 
 export interface YjsProviders {
   ydoc: Y.Doc;