Adds a MimePart tree model, parses it from the IMAP BODYSTRUCTURE when fetching the email body, caches it in a new mime_tree_json column (schema v28), and exposes a 'Show Mail Structure' overflow menu item that renders the indented tree (content-type, filename, size, encoding) in an AlertDialog alongside the existing headers dialog. Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>