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>
