feat(editor): path preview line, transform undo, complete undo coverage

- River/road tool now shows translucent blue path preview ribbon with
  edge lines between start and end points before applying
- Preview follows cursor when waiting for end point, locks when set
- Undo support for all remaining operations: rotateTerrain90, mirrorX,
  mirrorY, scaleHeights, offsetHeights, invertHeights, smoothBeaches
- Every terrain-modifying operation in the editor is now undoable

tools/editor/editor_ui.hpp

@@ -31,6 +31,7 @@ public:
     glm::vec3 getPathStart() const { return pathStart_; }
     glm::vec3 getPathEnd() const { return pathEnd_; }
     bool isPathReady() const { return pathStartSet_ && pathEndSet_; }
+    float getPathWidth() const { return pathWidth_; }
     void clearPath() { pathStartSet_ = false; pathEndSet_ = false; pathCapture_ = PathCapture::None; }
 
 private: