Add parts screen with per-part view and save

Adds a `stateParts` screen (v from message view) listing all MIME leaf
parts in a table with content-type, name, and size. Navigation with
↑↓/j/k, Enter to render text/* parts in a scrollable viewport
(statePartView), s to save any part to the current directory with a
prompted filename defaulting to the part's name.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

message.go

@@ -18,6 +18,91 @@ import (
 	"golang.org/x/text/encoding/ianaindex"
 )
 
+// MessagePart holds metadata and raw decoded bytes for a single MIME leaf part.
+type MessagePart struct {
+	Index     int
+	MediaType string
+	CT        string // full Content-Type header value (for charset info)
+	Name      string
+	Size      int
+	Data      []byte
+}
+
+// extractParts parses raw EML and returns a flat list of all MIME leaf parts.
+func extractParts(raw string) []MessagePart {
+	msg, err := mail.ReadMessage(strings.NewReader(raw))
+	if err != nil {
+		return nil
+	}
+	ct := msg.Header.Get("Content-Type")
+	cte := strings.ToLower(strings.TrimSpace(msg.Header.Get("Content-Transfer-Encoding")))
+	cd := msg.Header.Get("Content-Disposition")
+	var parts []MessagePart
+	counter := 0
+	collectParts(ct, cte, cd, msg.Body, &parts, &counter)
+	return parts
+}
+
+func collectParts(ct, cte, cd string, r io.Reader, out *[]MessagePart, n *int) {
+	mediaType, params, err := mime.ParseMediaType(ct)
+	if err != nil {
+		mediaType = "text/plain"
+		params = map[string]string{}
+	}
+
+	var decoded io.Reader
+	switch cte {
+	case "quoted-printable":
+		decoded = quotedprintable.NewReader(r)
+	case "base64":
+		decoded = base64.NewDecoder(base64.StdEncoding, r)
+	default:
+		decoded = r
+	}
+
+	if strings.HasPrefix(mediaType, "multipart/") {
+		boundary := params["boundary"]
+		if boundary == "" {
+			return
+		}
+		mr := multipart.NewReader(decoded, boundary)
+		for {
+			p, err := mr.NextPart()
+			if err != nil {
+				break
+			}
+			partCT := p.Header.Get("Content-Type")
+			partCTE := strings.ToLower(strings.TrimSpace(p.Header.Get("Content-Transfer-Encoding")))
+			partCD := p.Header.Get("Content-Disposition")
+			collectParts(partCT, partCTE, partCD, p, out, n)
+		}
+		return
+	}
+
+	data, _ := io.ReadAll(decoded)
+	*n++
+	*out = append(*out, MessagePart{
+		Index:     *n,
+		MediaType: mediaType,
+		CT:        ct,
+		Name:      partName(params, cd),
+		Size:      len(data),
+		Data:      data,
+	})
+}
+
+// savePart writes data to the named file in the current working directory.
+func savePart(data []byte, name string) (string, error) {
+	if name == "" {
+		name = "attachment"
+	}
+	path := filepath.Base(name)
+	if err := os.WriteFile(path, data, 0600); err != nil {
+		return "", fmt.Errorf("write %s: %w", path, err)
+	}
+	return path, nil
+}
+
 var wordDecoder = &mime.WordDecoder{
 	CharsetReader: func(charset string, input io.Reader) (io.Reader, error) {
 		// Fast path for the most common legacy charsets