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_diag4_test.go

@@ -0,0 +1,78 @@
+package internal
+
+import (
+	"bytes"
+	"encoding/binary"
+	"fmt"
+	"io"
+	"os"
+	"strings"
+	"testing"
+)
+
+func TestTNEFRawScan(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 {
+		data, _ := io.ReadAll(att.Data)
+		if strings.ToLower(att.Filename) != "winmail.dat" {
+			continue
+		}
+
+		fmt.Printf("TNEF raw size: %d bytes\n", len(data))
+
+		// Verify signature
+		if len(data) < 6 {
+			t.Fatal("too short")
+		}
+		sig := binary.LittleEndian.Uint32(data[0:4])
+		key := binary.LittleEndian.Uint16(data[4:6])
+		fmt.Printf("Signature: 0x%08X  Key: 0x%04X\n", sig, key)
+
+		offset := 6
+		attrNum := 0
+		for offset < len(data) {
+			if offset+9 > len(data) {
+				fmt.Printf("  Truncated at offset %d\n", offset)
+				break
+			}
+
+			level := data[offset]
+			attrID := binary.LittleEndian.Uint32(data[offset+1 : offset+5])
+			attrLen := binary.LittleEndian.Uint32(data[offset+5 : offset+9])
+
+			levelStr := "MSG"
+			if level == 0x02 {
+				levelStr = "ATT"
+			}
+
+			fmt.Printf("  [%03d] offset=%-8d level=%s id=0x%08X len=%d\n",
+				attrNum, offset, levelStr, attrID, attrLen)
+
+			// Move past: level(1) + id(4) + len(4) + data(attrLen) + checksum(2)
+			offset += 1 + 4 + 4 + int(attrLen) + 2
+
+			attrNum++
+			if attrNum > 200 {
+				fmt.Println("  ... stopping at 200 attributes")
+				break
+			}
+		}
+	}
+}