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/tnef_diag_test.go

@@ -0,0 +1,79 @@
+package internal
+
+import (
+	"bytes"
+	"fmt"
+	"io"
+	"os"
+	"strings"
+	"testing"
+
+	"github.com/teamwork/tnef"
+)
+
+func TestTNEFDiag(t *testing.T) {
+	testFile := `H:\Dev\Gits\EMLy\EML_TNEF.eml`
+	if _, err := os.Stat(testFile); os.IsNotExist(err) {
+		t.Skip("test EML file not present")
+	}
+
+	f, _ := os.Open(testFile)
+	defer f.Close()
+
+	// Parse the PEC outer envelope
+	outerEmail, err := Parse(f)
+	if err != nil {
+		t.Fatalf("parse outer: %v", err)
+	}
+
+	// Find postacert.eml
+	var innerData []byte
+	for _, att := range outerEmail.Attachments {
+		if strings.Contains(strings.ToLower(att.Filename), "postacert.eml") {
+			innerData, _ = io.ReadAll(att.Data)
+			break
+		}
+	}
+	if innerData == nil {
+		t.Fatal("no postacert.eml found")
+	}
+
+	// Parse inner email
+	innerEmail, err := Parse(bytes.NewReader(innerData))
+	if err != nil {
+		t.Fatalf("parse inner: %v", err)
+	}
+
+	fmt.Printf("Inner attachments: %d\n", len(innerEmail.Attachments))
+	for i, att := range innerEmail.Attachments {
+		data, _ := io.ReadAll(att.Data)
+		fmt.Printf("  [%d] filename=%q contentType=%q size=%d\n", i, att.Filename, att.ContentType, len(data))
+
+		if strings.ToLower(att.Filename) == "winmail.dat" ||
+			strings.Contains(strings.ToLower(att.ContentType), "ms-tnef") {
+
+			fmt.Printf("  Found TNEF! First 20 bytes: %x\n", data[:min(20, len(data))])
+			fmt.Printf("  isTNEFData: %v\n", isTNEFData(data))
+
+			decoded, err := tnef.Decode(data)
+			if err != nil {
+				fmt.Printf("  TNEF decode error: %v\n", err)
+				continue
+			}
+			fmt.Printf("  TNEF Body len: %d\n", len(decoded.Body))
+			fmt.Printf("  TNEF BodyHTML len: %d\n", len(decoded.BodyHTML))
+			fmt.Printf("  TNEF Attachments: %d\n", len(decoded.Attachments))
+			for j, ta := range decoded.Attachments {
+				fmt.Printf("    [%d] title=%q size=%d\n", j, ta.Title, len(ta.Data))
+			}
+			fmt.Printf("  TNEF Attributes: %d\n", len(decoded.Attributes))
+		}
+	}
+}
+
+func min(a, b int) int {
+	if a < b {
+		return a
+	}
+	return b
+}