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

ts_install/download-schematron.ts

@@ -57,8 +57,12 @@ async function main() {
   console.log('\n✅ Schematron download complete!');
 }
 
-// Run the script
-main().catch(error => {
-  console.error('❌ Script failed:', error);
-  process.exit(1);
-});
+// Run if executed directly
+if (import.meta.url === `file://${process.argv[1]}`) {
+  main().catch(error => {
+    console.error('❌ Script failed:', error);
+    process.exit(1);
+  });
+}
+
+export default main;