POS and Ledger App for Building Materials Shops

I run two building-material shops (sand, bricks, cement, steel/TMT) in Delhi NCR. I need a reliable POS and ledger app for daily use at the counter. The data of both shops will be separate.

IMPORTANT: I already have a working, tested prototype (a single HTML file, about 1,500 lines of plain JavaScript) and a full written specification of every screen, flow and business rule. This is NOT a from-scratch build. You will turn the prototype into a proper, reliable app. I will share the prototype and the spec after we talk.

What the app does (already in the prototype):
- Fast 5-step sale flow: choose items (with search), quantity/rate, customer, payment, delivery
- Weight-based items (saria sold in bundles, priced per kg, each bundle weighed)
- Customer credit (udhaar): payments always clear the oldest bill first, ledger, statement, aging report
- Stock tracking with low-stock alerts, purchases, suppliers, expenses, returns, deliveries
- Cash reconciliation, dashboard and date-range reports
- Optional GST per sale, labour charges

What I need from you:
1. Move the app off one browser's local storage to a proper database (for example Supabase/PostgreSQL), with login.
2. Reliable multi-device sync for 2 shops, working on a Windows PC/laptop, a fixed tablet, and Android phones. It must work offline and sync when internet returns. Two people selling at the same time must never lose entries, double-count a payment or create the same invoice number twice.
3. Automatic cloud backup with daily history and easy restore.
4. Professional printable/shareable bill (PDF) with the name "Sunny Traders", amounts shown as "Rs.", previous balance, labour and GST.
5. PIN lock, plus optional separate staff access with limited permissions.
6. CSV/PDF export of reports.
7. Fix any bugs you find. Stock and balances must always be calculated from transactions and never go out of sync.
8. Simple enough that my staff can learn it in under an hour. Keep the current sale flow speed.

Deliverables:
- Complete source code (I own it) and a working hosted app on accounts created in MY name
- Automated tests for the money and stock rules (oldest-bill-first payment allocation, stock never negative, return limits, no duplicate invoice numbers)
- Short user guide and a hand-off session on a call
- 1 month of free bug-fix support after launch

Please tell me about similar billing, inventory or ledger apps you have built, and send questions about my spec; I prefer people who ask questions.