test: add certificate feature unit tests and null-safety fix
Add comprehensive unit tests for the certificate upload, export,
and detail management feature:
- CertificateExportDialog: 21 tests covering format selection,
blob download, error handling, and password-protected exports
- CertificateUploadDialog: 23 tests covering file validation,
format detection, drag-and-drop, and upload flow
- CertificateDetailDialog: 19 tests covering detail display,
loading state, missing fields, and branch coverage
- CertificateChainViewer: 8 tests covering chain visualization
- CertificateValidationPreview: 16 tests covering validation display
- FileDropZone: 18 tests covering drag-and-drop interactions
- useCertificates hooks: 10 tests covering all React Query hooks
- certificates API: 7 new tests for previously uncovered endpoints
Fix null-safety issue in ProxyHosts where cert.domains could be
undefined, causing a runtime error on split().
Frontend patch coverage: 90.6%, overall lines: 89.09%