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`.

backend/utils/mail/eml_reader.go

@@ -146,6 +146,9 @@ func ReadEmlFile(filePath string) (*EmailData, error) {
 		})
 	}
 
+	// Expand any TNEF (winmail.dat) attachments into their contained files.
+	attachments = expandTNEFAttachments(attachments)
+
 	isPec := hasDatiCert && hasSmime
 
 	// Format From
@@ -267,6 +270,9 @@ func ReadPecInnerEml(filePath string) (*EmailData, error) {
 		})
 	}
 
+	// Expand any TNEF (winmail.dat) attachments into their contained files.
+	attachments = expandTNEFAttachments(attachments)
+
 	isPec := hasDatiCert && hasSmime
 
 	// Format From