feat(editor): undo object placement, snap to ground, keyboard shortcuts

- Ctrl+Z in Object/NPC mode undoes last placement (50-deep stack)
- "Snap Ground" button raycasts straight down to place object on terrain
- Useful for objects placed too high or moved off terrain surface
- Undo stack adjusts indices when objects are removed mid-stack

tools/editor/editor_app.hpp

@@ -61,6 +61,7 @@ public:
 
     void startGizmoMode(TransformMode mode);
     void setGizmoAxis(TransformAxis axis);
+    void snapSelectedToGround();
     TransformGizmo& getGizmo() { return viewport_.getGizmo(); }
     bool shouldOpenContextMenu() const { return openContextMenu_; }
     void clearContextMenuFlag() { openContextMenu_ = false; }