Add PayPal webhook handler and marketing styles

- Implemented PayPal webhook for handling payment notifications, including signature verification and transaction updates.
- Created invoice generation and license management for software upgrades upon successful payment.
- Added comprehensive logging for debugging purposes.
- Introduced new CSS styles for the marketing file management system, including layout, toolbar, breadcrumb navigation, search filters, and file management UI components.

products_software_upgrade_paths_manage.php

@@ -28,7 +28,7 @@ $path = [
     'from_version_id' => '',
     'to_version_id' => '',
     'price' => '',
-    'currency' => 'USD',
+    'currency' => 'EUR',
     'description' => '',
     'is_active' => 1,
     'created' => '',