feat(postinstall): Add robust postinstall resource download with version checking

- Implemented version checking to avoid unnecessary re-downloads
- Added retry logic with 3 attempts for network failures
- Included network connectivity detection
- Graceful error handling that never fails the install
- Support for CI environment variable (EINVOICE_SKIP_RESOURCES)
- Successfully tested all functionality including version checking and error handling

assets/schematron/.version

@@ -0,0 +1 @@
+1.0.0

assets/schematron/EN16931-CII-v1.3.14.meta.json

@@ -3,5 +3,5 @@
   "version": "1.3.14",
   "url": "https://github.com/ConnectingEurope/eInvoicing-EN16931/raw/master/cii/schematron/EN16931-CII-validation.sch",
   "format": "CII",
-  "downloadDate": "2025-08-11T11:05:40.209Z"
+  "downloadDate": "2025-08-12T05:01:07.919Z"
 }

assets/schematron/EN16931-EDIFACT-v1.3.14.meta.json

@@ -3,5 +3,5 @@
   "version": "1.3.14",
   "url": "https://github.com/ConnectingEurope/eInvoicing-EN16931/raw/master/edifact/schematron/EN16931-EDIFACT-validation.sch",
   "format": "CII",
-  "downloadDate": "2025-08-11T11:05:40.547Z"
+  "downloadDate": "2025-08-12T05:01:08.258Z"
 }

assets/schematron/EN16931-UBL-v1.3.14.meta.json

@@ -3,5 +3,5 @@
   "version": "1.3.14",
   "url": "https://github.com/ConnectingEurope/eInvoicing-EN16931/raw/master/ubl/schematron/EN16931-UBL-validation.sch",
   "format": "UBL",
-  "downloadDate": "2025-08-11T11:05:39.868Z"
+  "downloadDate": "2025-08-12T05:01:07.598Z"
 }

assets/schematron/PEPPOL-EN16931-UBL-v3.0.17.meta.json

@@ -3,5 +3,5 @@
   "version": "3.0.17",
   "url": "https://github.com/OpenPEPPOL/peppol-bis-invoice-3/raw/master/rules/sch/PEPPOL-EN16931-UBL.sch",
   "format": "UBL",
-  "downloadDate": "2025-08-11T11:05:40.954Z"
+  "downloadDate": "2025-08-12T05:01:08.635Z"
 }

assets/schematron/xrechnung/temp/xrechnung-schematron.zip

@@ -1 +0,0 @@
-404: Not Found