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-test-samples.ts

@@ -202,4 +202,5 @@ if (import.meta.url === `file://${process.argv[1]}`) {
   main().catch(console.error);
 }
 
-export { downloadTestSamples, TEST_SAMPLE_SOURCES };
+export default main;
+export { TEST_SAMPLE_SOURCES };