User story

As a workflow developer, I want canvas controls for undo/redo, so that I can revert and reapply workflow changes.

Details

Canvas controls are mostly implemented, but undo/redo are empty stubs.

Partially Implemented:

• ❌ Undo button exists but function is EMPTY STUB (line 105, 559-561)
• ❌ Redo button exists but function is EMPTY STUB (line 106, 562-564)
• ❌ Keyboard shortcuts registered but not functional (line 483-503)

User acceptance criteria

• Undo button reverts last workflow change
• Redo button reapplies undone change
• Cmd/Ctrl+Z triggers undo
• Cmd/Ctrl+Y or Cmd/Ctrl+Shift+Z triggers redo
• Undo/redo affects all workflow changes (nodes, edges, positions)
• Button states reflect undo/redo availability
• Works correctly in collaborative editing scenarios