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-xrechnung-rules.ts

@@ -171,8 +171,12 @@ async function downloadXRechnungRules(): Promise<void> {
   console.log('3. Add XRechnung-specific TypeScript validators');
 }
 
-// Run the script
-downloadXRechnungRules().catch(error => {
-  console.error('Failed to download XRechnung rules:', error);
-  process.exit(1);
-});
+// Run if executed directly
+if (import.meta.url === `file://${process.argv[1]}`) {
+  downloadXRechnungRules().catch(error => {
+    console.error('Failed to download XRechnung rules:', error);
+    process.exit(1);
+  });
+}
+
+export default downloadXRechnungRules;