Add billing page with payment history and invoice download (#691)

* 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

README.md

@@ -127,6 +127,7 @@ The following list contains such features - they are only available for [etke.cc
 * 📬 [Server Notifications indicator and page](https://github.com/etkecc/synapse-admin/pull/240)
 * 🛠️ [Server Commands panel](https://github.com/etkecc/synapse-admin/pull/365)
 * 🚀 [Server Actions page](https://github.com/etkecc/synapse-admin/pull/457)
+* 💳 [Billing page](https://github.com/etkecc/synapse-admin/pull/691)
 
 ### Development