Role-Based Auth for Billing App -- 2

I’ve already built a full-featured billing and inventory web app for ASK INDUSTRIAL WORKS with vanilla HTML, CSS, JavaScript, and localStorage. Now I need a lean authentication layer so different users see only what they should.

Scope
• Add a simple login screen that stores credentials locally (no external database—keep using localStorage).
• Create three roles: Admin, Manager, Staff.
• Apply access rules:
– Admin: everything.
– Manager: approve payments, generate reports, manage products and inventory.
– Staff: day-to-day billing and stock entries only.
• Hide or disable UI elements based on the active role.
• Maintain the responsive, printable layout and existing reporting logic.
• Deliver clean, well-commented code plus a short README so I can extend it later.

Tech constraints
Pure JavaScript front-end, Python only if you need a lightweight helper script—but no new frameworks or back-end services.

If you’ve implemented lightweight role-based security in front-end apps before, I’d love to see a quick example when you bid.