going to localhost:5000 give the backend but i cant access the front end when going to localhost:3000. Find the error.
#PORT=5000
JWT_SECRET=your_jwt_secret_key
BACKEND_URL=http://localhost:5000

// App.js
import React from 'react';
import { AuthProvider } from './context/AuthContext';
import AppNavigator from './navigation/AppNavigator';

const App = () => {
    return (
        <AuthProvider>
            <AppNavigator />
        </AuthProvider>
    );
};

export default App;
// package.json
{
  "name": "fitness-app-backend",
  "version": "1.0.0",
  "main": "index.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "keywords": [],
  "author": "",
  "license": "ISC",
  "description": "",
  "dependencies": {
    "@react-native-community/masked-view": "^0.1.11",
    "@react-native-picker/picker": "^2.8.1",
    "@react-navigation/native": "^6.1.18",
    "@react-navigation/stack": "^6.4.1",
    "axios": "^1.7.7",
    "bcryptjs": "^2.4.3",
    "cors": "^2.8.5",
    "dotenv": "^16.4.5",
    "express": "^4.21.0",
    "jsonwebtoken": "^9.0.2",
    "mongoose": "^8.7.0",
    "react-native-chart-kit": "^6.12.0",
    "react-native-dotenv": "^3.4.11",
    "react-native-gesture-handler": "^2.20.0",
    "react-native-multiple-select": "^0.5.12",
    "react-native-reanimated": "^3.15.3",
    "react-native-safe-area-context": "^4.11.0",
    "react-native-screens": "^3.34.0",
    "react-native-svg": "^15.7.1"
  }
}
// server.js
require('dotenv').config();
const express = require('express');
const mongoose = require('mongoose');
const cors = require('cors');

const app = express();

// Middleware
app.use(express.json());

// CORS Configuration
app.use(cors({
    origin: 'http://localhost:3000', // Frontend URL
    methods: ['GET', 'POST', 'PUT', 'DELETE'],
    credentials: true,
}));

// Connect to MongoDB
mongoose.connect(process.env.MONGO_URI, {
    // Removed deprecated options as per earlier advice
})
.then(() => console.log('MongoDB Connected'))
.catch(err => console.log(err));

// Routes
app.get('/', (req, res) => {
    res.send('Fitness App Backend');
});

const authRoutes = require('./routes/auth');
app.use('/api/auth', authRoutes);

const exerciseRoutes = require('./routes/exercises');
app.use('/api/exercises', exerciseRoutes);

const progressRoutes = require('./routes/progress');
app.use('/api/progress', progressRoutes);

// Start Server
const PORT = process.env.PORT || 5000;
app.listen(PORT, () => {
    console.log(`Server running on port ${PORT}`);
});{
  "name": "fitness-app-backend",
  "version": "1.0.0",
  "main": "index.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "keywords": [],
  "author": "",
  "license": "ISC",
  "description": "",
  "dependencies": {
    "@react-native-community/masked-view": "^0.1.11",
    "@react-native-picker/picker": "^2.8.1",
    "@react-navigation/native": "^6.1.18",
    "@react-navigation/stack": "^6.4.1",
    "axios": "^1.7.7",
    "bcryptjs": "^2.4.3",
    "cors": "^2.8.5",
    "dotenv": "^16.4.5",
    "express": "^4.21.0",
    "jsonwebtoken": "^9.0.2",
    "mongoose": "^8.7.0",
    "react-native-chart-kit": "^6.12.0",
    "react-native-dotenv": "^3.4.11",
    "react-native-gesture-handler": "^2.20.0",
    "react-native-multiple-select": "^0.5.12",
    "react-native-reanimated": "^3.15.3",
    "react-native-safe-area-context": "^4.11.0",
    "react-native-screens": "^3.34.0",
    "react-native-svg": "^15.7.1"
  }
}