royalsuthan06/sparkbill_apk ? reverse-engineered prompt

Build me an offline first billing and inventory app for a small shop, using Flutter, that works on Android, iPhone, Windows, macOS, Linux, and the web from the same codebase.

I want a clean adaptive layout, with a simple mobile view and a more desktop friendly layout on wider screens. The app should let me add products, edit or remove them, search items quickly while making a bill, adjust quantities easily, and keep a running cart. It should save everything locally so it still works without internet, including products, bills, and sales history.

Please also include invoice generation with a nice printable PDF receipt, plus a way to print or reprint old bills. I want a reports screen that shows daily, weekly, yesterday, and custom date range sales summaries, with totals and bill counts. Make the inventory screen support filters and sorting too.

Use the existing project structure if it helps, and look up current Flutter docs online if you need to.