BlakvGhost/Neptune — reverse-engineered prompt

Build me a stock management web app called Neptune for small shop owners. I want an admin to log in with a phone number and password, manage different shops, add and update products, track quantities, and see what is in stock at any time. The app should also handle simple automatic billing so staff can create bills for customers from the items in inventory and have the stock update after a sale.

Keep the screens clean and practical, with forms and tables that are easy to use. Use PHP with CodeIgniter4, MySQL, HTML, CSS, JavaScript, jQuery and Ajax like the original project. Include a database setup file and clear local setup instructions so I can run it with php spark serve or host it under the public folder on Apache. Look up current docs online if you need to.