feat(editor): random rotation, placed object list, quality of life

- Random Rotation checkbox: each placed object gets a random Y rotation
  (great for natural-looking tree/rock placement without manual tweaking)
- Placed Object List: collapsible list in Object panel showing all placed
  objects with name and position, click to select
- Both features reduce repetitive manual work when building dense zones

tools/editor/object_placer.cpp

@@ -25,7 +25,13 @@ void ObjectPlacer::placeObject(const glm::vec3& position) {
     obj.nameId = 0;
     obj.uniqueId = nextUniqueId();
     obj.position = position;
-    obj.rotation = glm::vec3(0.0f, placementRotY_, 0.0f);
+    float rotY = placementRotY_;
+    if (randomRotation_) {
+        static std::mt19937 rng(42);
+        std::uniform_real_distribution<float> dist(0.0f, 360.0f);
+        rotY = dist(rng);
+    }
+    obj.rotation = glm::vec3(0.0f, rotY, 0.0f);
     obj.scale = placementScale_;
     obj.selected = false;