Update app.js with optimized code structure

[Mugunth140]

This optimized Express.js server configuration enhances scalability, readability, and professionalism by applying modern best practices. It includes middleware for handling CORS, cookie parsing, and request body parsing. Environment variables are cleanly managed using dotenv, keeping sensitive information secure. All API routes for users, appointments, medicines, cart management, payments, and testimonials are structured with clear paths, allowing for modular routing and future scalability. A default route ensures the server responds to base URL requests, and a global error handler is implemented to provide consistent and centralized error management. This setup is ideal for a production-ready application, designed for efficient backend API management.

[vercel]

Deployment failed with the following error:

Resource is limited - try again in 5 hours (more than 100, code: "api-deployments-free-per-day").

[github-actions]

Welcome to our repository. 🎊

Thank you so much for taking the time to point this out and for your contribution. @itsmohit097 will check this PR as soon as possible. Thank you for your cooperation.