- Add 'Open Register' modal with register number and opening amount inputs
- loadRegister now shows clickable warning when no register is open
- checkout() opens register modal instead of plain alert when no register
- Add openRegister() API call to POST /pos/api/register/open
- Expose showOpenRegisterModal, closeOpenRegisterModal, openRegister globally
- Add cache-bust query params to pos.css and pos.js