The Problem Every freelancer needs invoices. Most "free" tools aren't really free — they limit you to 5 invoices/month, require an account, or add watermarks. I wanted to build the simplest possible invoice tool with zero ongoing costs. The Architecture The entire application runs client-side in the browser: No backend — all logic runs in JavaScript No database — form state lives in the DOM No auth — no accounts to manage No server costs — Firebase Hosting free tier handles everything Key Technical Decisions PDF Generation: I used jsPDF to generate invoices client-side. The PDF is created entirely in the browser and downloaded directly — no server round-trip needed. Offline Support: The app is a Progressive Web App (PWA) with a service worker. Once loaded, it works without an internet connection. Privacy: Since there's no backend, user data never leaves the browser. No cookies, no tracking beyond basic Firebase Analytics.…