Scalable Cross-Platform App Build

I’m kick-starting a cross-platform application that must feel native on Android, iOS and laptop browsers from day one, yet stay light enough to scale to a very large user base later. I need you to take care of the entire lifecycle—clean UI/UX, robust backend, an intuitive admin panel, store submission, and post-launch support.

Core feature set
The first milestone revolves around secure user accounts. I want classic email-and-password login hardened with the following:

• Two-factor authentication
• Self-service password-recovery flows
• End-to-end data encryption (in transit and at rest)

Everything should be architected in a way that lets me bolt on payments, social logins or sharing modules down the line without rewrites.

What I’d like from you
1. A link to your portfolio plus any live apps that show similar secure sign-in flows.
2. A broad timeline and cost estimate broken into sensible milestones (design, backend, testing, launch, support).
3. Your recommended tech stack—Flutter, React Native, Swift/Kotlin, Firebase, AWS, or anything you feel suits high concurrency.

Deliverables I will review and sign off on include wireframes, clickable prototypes, documented APIs, a working admin dashboard, deploy scripts, and successful Play Store rollout.

If this aligns with your expertise, send over the details above and let’s move quickly to the planning stage.