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>

msgs.go

@@ -19,6 +19,8 @@ type messageLoadedMsg struct {
 type messageErrMsg struct{ err error }
 type savedMsg struct{ path string }
 type saveErrMsg struct{ err error }
+type partSavedMsg struct{ path string }
+type partSaveErrMsg struct{ err error }
 
 // ── Tea commands ──────────────────────────────────────────────────────────────
 
@@ -61,3 +63,13 @@ func saveCmd(id, content string) tea.Cmd {
 		return savedMsg{path}
 	}
 }
+
+func savePartCmd(data []byte, name string) tea.Cmd {
+	return func() tea.Msg {
+		path, err := savePart(data, name)
+		if err != nil {
+			return partSaveErrMsg{err}
+		}
+		return partSavedMsg{path}
+	}
+}