E-Commerce Web App with Square -- 2

I’m ready to move an e-commerce concept from mock-ups to a working web application and the priority is flawless Square payment processing. The build should cover the full purchase flow—cart, checkout, and post-purchase notifications—while keeping the codebase clean and maintainable.

The essentials
• Integrate Square’s latest API for one-time payments, refunds, and webhooks, following PCI-DSS best practices.
• Develop a responsive storefront where products, prices, and inventory can be managed from an admin panel I can grow into later.
• Implement secure customer and order data storage, with server-side validation and error handling surfaced elegantly in the UI.
• Provide environment-specific configs so I can swap between the Square sandbox and live credentials without redeploying.

Optional later phases could include user accounts, product search, or adding PayPal or Stripe alongside Square, so a modular architecture is important.

Acceptance criteria
1. I can load the site, add a test item to the cart, pay via a Square sandbox card, and see confirmation in both the UI and my Square dashboard.
2. All sensitive keys are stored outside the repo and injected through environment variables.
3. A brief README explains setup, build, and deployment steps.

Please use modern, widely-supported tech (React/Next.js, Vue/Nuxt, or similar on the front end and Node, Django, or Laravel on the back end are all fine). Clean commits and concise inline comments will make maintenance easier for me.