Phase 2: Scene Undo/Redo System

[fernandotonon]

Goal

Enable undo/redo for all scene operations using Qt's QUndoStack.

• UndoManager singleton wrapping QUndoStack
• Command classes: TranslateCommand, RotateCommand, ScaleCommand, DeleteCommand, CreateCommand
• Capture state in mousePressEvent(), push in mouseReleaseEvent()
• Edit menu + Ctrl+Z / Ctrl+Shift+Z
• Expose undo/redo MCP tools

Depends on: Phase 1 (needs scale operations to wrap)
Part of: UX Redesign Roadmap