Add revision history to the editor

[priethor]

What

Closes #137.

Cortext now has a History panel in the editor. You can open it from the document toolbar or the sidebar row menu, preview an older version without editing it by accident, see what changed in the content and properties, and restore that version. When you restore, the version you were on is kept in history too.

How

• Uses WordPress 7.0 revision support for document fields and layout data.
• Saves the current document before applying an older revision.
• Disables restore while the document has unsaved changes, is saving, or is still in Trash.

Testing Instructions

1. Create or open a Cortext document.
2. Save one version, then change the title, body, or properties and save again.
3. Open History from the top bar.
4. Select an older revision and confirm the editor switches to read-only preview mode.
5. Turn on Show changes and check the content/property diff.
6. Close the History panel and confirm you're back in the normal editor.
7. Restore an older revision and confirm the previous current version is still listed in history.