* Add billing page with payment history and invoice download
- Add new BillingPage component with MUI table for payment display
- Add billing menu item to AdminLayout with Payment icon
- Add getPayments and getInvoice methods to dataProvider
- Implement invoice download functionality with proper error handling
- Add routing for /billing path
- Support for scheduler/{hash}/payments API endpoint
- Enhanced error handling for 500, 404, 401, 403 HTTP errors
- Loading states and user feedback for better UX
* update readme; add docs; small visual changes; make it fail on payments API errors
