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_diag8_test.go

@@ -0,0 +1,97 @@
+package internal
+
+import (
+	"bytes"
+	"encoding/binary"
+	"fmt"
+	"io"
+	"os"
+	"strings"
+	"testing"
+
+	"github.com/teamwork/tnef"
+)
+
+func TestTNEFDeepAttachment(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()
+
+	outerEmail, _ := Parse(f)
+	var innerData []byte
+	for _, att := range outerEmail.Attachments {
+		if strings.Contains(strings.ToLower(att.Filename), "postacert.eml") {
+			innerData, _ = io.ReadAll(att.Data)
+			break
+		}
+	}
+
+	innerEmail, _ := Parse(bytes.NewReader(innerData))
+	for _, att := range innerEmail.Attachments {
+		rawData, _ := io.ReadAll(att.Data)
+		if strings.ToLower(att.Filename) != "winmail.dat" {
+			continue
+		}
+
+		// Dig to level 2: top → embedded[0] → embedded[0]
+		streams0 := findEmbeddedTNEFStreams(rawData)
+		if len(streams0) == 0 {
+			t.Fatal("no embedded streams at level 0")
+		}
+		streams1 := findEmbeddedTNEFStreams(streams0[0])
+		if len(streams1) == 0 {
+			t.Fatal("no embedded streams at level 1")
+		}
+
+		// Decode level 2
+		decoded2, err := tnef.Decode(streams1[0])
+		if err != nil {
+			t.Fatalf("level 2 decode: %v", err)
+		}
+
+		fmt.Printf("Level 2 attachments: %d\n", len(decoded2.Attachments))
+		for i, a := range decoded2.Attachments {
+			fmt.Printf("  [%d] title=%q size=%d\n", i, a.Title, len(a.Data))
+			if len(a.Data) > 20 {
+				fmt.Printf("      first 20 bytes: %x\n", a.Data[:20])
+				// Check for EML, MSG, TNEF signatures
+				if len(a.Data) >= 4 {
+					sig := binary.LittleEndian.Uint32(a.Data[0:4])
+					if sig == 0x223E9F78 {
+						fmt.Println("      -> TNEF stream!")
+					}
+				}
+				if len(a.Data) >= 8 && bytes.Equal(a.Data[:8], []byte{0xD0, 0xCF, 0x11, 0xE0, 0xA1, 0xB1, 0x1A, 0xE1}) {
+					fmt.Println("      -> MSG (OLE2) file!")
+				}
+				// Check if text/EML
+				if a.Data[0] < 128 && a.Data[0] >= 32 {
+					preview := string(a.Data[:min2(200, len(a.Data))])
+					if strings.Contains(preview, "From:") || strings.Contains(preview, "Content-Type") || strings.Contains(preview, "MIME") || strings.Contains(preview, "Received:") {
+						fmt.Printf("      -> Looks like an EML file! First 200 chars: %s\n", preview)
+					} else {
+						fmt.Printf("      -> Text data: %.200s\n", preview)
+					}
+				}
+			}
+		}
+
+		// Also check level 2's attAttachment for embedded msgs
+		streams2 := findEmbeddedTNEFStreams(streams1[0])
+		fmt.Printf("\nLevel 2 embedded TNEF streams: %d\n", len(streams2))
+
+		// Check all MAPI attributes at level 2
+		fmt.Println("\nLevel 2 MAPI attributes:")
+		for _, attr := range decoded2.Attributes {
+			fmt.Printf("  0x%04X: %d bytes\n", attr.Name, len(attr.Data))
+			// PR_BODY
+			if attr.Name == 0x1000 && len(attr.Data) < 500 {
+				fmt.Printf("    PR_BODY: %s\n", string(attr.Data))
+			}
+		}
+	}
+}