Add TNEF handling and email loading improvements

- Implement TNEF extraction and recursive parsing in new `tnef_reader.go` and associated tests.
- Create tests for TNEF extraction scenarios in `tnef_diag_test.go`, `tnef_diag7_test.go`, and `tnef_diag8_test.go`.

frontend/src/lib/utils/mail/index.ts

@@ -33,6 +33,7 @@ export {
   getEmailFileType,
   isEmailFile,
   loadEmailFromPath,
+  loadEmailFromPathLegacy,
   openAndLoadEmail,
   processEmailBody,
   type LoadEmailResult,