feat(editor): add undo/redo buttons to viewer toolbar

[rkolpakov-sudo]

What does this PR do?

Adds Undo/Redo buttons to the viewer toolbar (top-left, next to the collapse and view-mode controls). The buttons subscribe to the history store via subscribeHistoryCommandState and dispatch through runUndo/runRedo, which respect the collaborative history delegate and fall back to standalone Zundo when no controller is installed. Buttons disable when nothing can be undone/redone.

Keyboard shortcuts were already wired (Ctrl/Cmd+Z undo, Ctrl/Cmd+Shift+Z redo) and command palette entries already existed; this PR adds the visible toolbar affordance and the CHANGELOG entry.

How to test

1. Run bun dev and open the editor at http://localhost:3002
2. Create or move a node in the 3D view
3. Click the Undo button — the action is reverted; click Redo — it is reapplied
4. With an empty history, both buttons are disabled
5. Press Ctrl/Cmd+Z and Ctrl/Cmd+Shift+Z to confirm the keyboard shortcuts still work alongside the buttons

Screenshots / screen recording

N/A — visual change; a screen recording will be added if requested.

Checklist

• I've tested this locally with bun dev
• My code follows the existing code style (run bun check to verify)
• I've updated relevant documentation (if applicable)
• This PR targets the main branch

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Note

Low Risk
UI-only addition that reuses existing history APIs; no changes to undo/redo or scene mutation logic.

Overview
Adds Undo and Redo controls to the top-left community viewer toolbar (between sidebar collapse and 3D/2D/split view mode).

A new HistoryControls block subscribes to subscribeHistoryCommandState, reads canUndo / canRedo via getHistoryCommandState, and invokes runUndo / runRedo so behavior matches the command palette and keyboard shortcuts (including collaborative history when a delegate is set). Buttons use the same toolbar styling as other controls and are visually disabled when history is empty. CHANGELOG documents the feature under Unreleased → Features.

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