fix: change backend port to 3000 to match frontend proxy

Author: 许君山

backend/server.js

@@ -3,7 +3,7 @@ import cors from 'cors';
 import { conjugate } from './conjugationEngine.js';
 
 const app = express();
-const PORT = process.env.PORT || 3001;
+const PORT = process.env.PORT || 3000;
 
 // 中间件
 app.use(cors());