feat: list and undo buttons in the document editor

Adds the toolbar controls for the list and undo support already in the
engine: bulleted list, numbered list, and an undo button that steps the
document back to how it was before the last save.

The list buttons toggle - pressing the same one again turns the
paragraphs back into ordinary text. A paragraph the document already
renders as a list item (inside a ul/ol) is recognised as one, so an
existing list is not flattened to body text by the next save. Undo
re-fetches and re-renders the restored document.

Verified end to end in the browser: a bulleted list persisted into the
saved .docx as a real Word list (w:numPr + numbering definitions
registered in the package), and undo restored the previous version
without destroying the later one.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018RCY6K4dNwdbo9gU3HQ9un