Improve TOC Bulk Actions UX: Shift+Click, One-Step Undo

[mettta]

This PR improves TOC collapse/expand usability and refactors the TOC script structure.

What was added

1. Shift+Click subtree bulk toggle

• Added Shift+Click on a TOC branch control to collapse/expand its whole subtree.
• Existing double-click subtree behavior is kept.

2. Top bulk controls panel

• Added two controls at the top of TOC: Collapse all and Expand all. They apply to the whole TOC tree in one click.

3. One-step undo for bulk actions

• Added one-step undo for the latest bulk operation.
• After a bulk click, that same button becomes Undo.
• If user clicks the other bulk button, Undo switches to that last clicked button.
• If user performs a normal tree action (non-bulk), Undo is cleared.
• Trivial snapshots (all branches in the same state) are not stored.