Payment Tool UI Redesign

Our in-house payment tool, built on a Python/Flask back-end with inline HTML templates, is fully functional but visually dated. I want the interface alone reworked into a clean, flat-design experience that feels modern, responds gracefully on mobile, and guides users through each step more intuitively.

You will receive a test login plus a reference mock-up so you can see the current screens and the visual direction we have in mind. All business logic, API calls, and security layers remain untouched; you will focus strictly on the front-end layer that sits inside the existing Flask view functions.

I need updated template files (HTML, minimal Jinja where required), CSS, and any lightweight JavaScript you add, dropped back into the repo so they render flawlessly with the live billing system. Please keep styling self-contained, observe best practices for accessibility, and verify that every screen scales from phone to desktop without layout breaks.

Acceptance is simple: once I can pull your branch, spin up the app, and walk through each payment step on desktop and mobile simulators without seeing visual glitches or flow confusion, the job is done.