docs: Write undo/redo and history documentation

[ggfevans]

Summary

Document the undo/redo system and command history in Rackula.

Content Outline

Undo/Redo Basics

• Undo with Ctrl+Z (Cmd+Z on Mac)
• Redo with Ctrl+Shift+Z or Ctrl+Y
• Toolbar undo/redo buttons

What Can Be Undone

• Place device
• Move device
• Remove device
• Update device properties (face, name)
• Update rack properties
• Clear rack
• Add/update/delete device types

Command History

• History tracking with descriptions
• Multiple undo steps supported
• Redo stack behaviour

Limitations

• History resets on major operations
• No undo for export or share
• Browser refresh clears history

Acceptance Criteria

• All undoable actions listed
• Keyboard shortcuts documented
• Limitations clearly stated

Test Requirements

• Build completes without errors