Add Authentication & Payment Features

I have an existing web app that is already live and stable; now I’m ready to move it forward by adding two key capabilities: complete user authentication and seamless payment integration.

For authentication, I need a solid, secure flow that can scale—email + password, social logins, or even two-factor are all on the table, so I’ll welcome your guidance on the approach that best fits modern security standards and keeps the UX smooth.

On the payments side, I want users to be able to check out inside the app without friction. I am open to whichever mainstream gateway (Stripe, PayPal, etc.) you feel is the most reliable for subscriptions and one-off purchases; what matters is PCI compliance, clear error handling, and clean API use.

You’ll work against the current codebase, keep existing functionality untouched, and deliver:

• Fully working user authentication flow
• Integrated payment gateway with test and live modes

I’ll provide repo access, staging credentials, and quick feedback as you progress. Let me know which frameworks or libraries you’d leverage and how you’ll ensure security and performance stay tight while these new features go in.