Phase 3: QML Properties Panel — Replace 4-Tab Sidebar

[fernandotonon]

Goal

Replace the cramped 4-tab widget sidebar with a scrollable, collapsible QML properties panel (like Godot/Unity inspector).

Layout

• Scene Outliner — always visible, tree view
• Transform — visible when object selected
• Material — visible when entity selected
• Primitive Parameters — procedural meshes only
• Animation — animated entities only

Implementation

• PropertiesPanel.qml with CollapsibleSection.qml component
• PropertiesPanelController QML_SINGLETON bridging Manager/SelectionSet/TransformOperator
• SceneTreeModel (QAbstractItemModel) for QML TreeView
• QQuickWidget (software rendering) in QDockWidget

Depends on: Nothing (can run in parallel with Phase 1)
Part of: UX Redesign Roadmap