fix(editor): Clear All, New Terrain reset, right-click menu, gizmo drag

- Clear All now actually removes all objects and NPCs (was only clearing
  selections before). Uses new ObjectPlacer::clearAll() method.
- New Terrain clears all objects/NPCs and resets viewport before creating
  fresh terrain. Fixes stale state from previous session.
- Right-click context menu works on both objects AND NPCs with
  appropriate options for each (Move/Rotate/Scale for objects,
  Fly To/Duplicate for NPCs)
- Gizmo drag: left-click now confirms the transform (ends drag) instead
  of requiring mouse-up. Right-click cancels. Camera no longer steals
  mouse events while gizmo is active.
- Right-click on unselected area passes through to camera correctly

tools/editor/object_placer.hpp

@@ -54,6 +54,7 @@ public:
     bool loadFromFile(const std::string& path);
 
     const std::vector<PlacedObject>& getObjects() const { return objects_; }
+    void clearAll() { objects_.clear(); undoStack_.clear(); selectedIdx_ = -1; }
     size_t objectCount() const { return objects_.size(); }
 
     float getPlacementRotationY() const { return placementRotY_; }