修复HierarchyPanel：空白区域右键菜单和拖拽到根目录功能

- 将右键菜单移到Child窗口内部，修复空白区域右键不弹出菜单的问题
- 添加InvisibleButton作为拖放目标，支持将实体拖到根级别

ui/src/panels/HierarchyPanel.cpp

@@ -28,11 +28,6 @@ void HierarchyPanel::Render() {
     
     std::string filter = m_searchBuffer;
     
-    if (ImGui::BeginPopupContextWindow("HierarchyContextMenu", ImGuiPopupFlags_MouseButtonRight | ImGuiPopupFlags_NoOpenOverItems)) {
-        RenderCreateMenu(INVALID_ENTITY);
-        ImGui::EndPopup();
-    }
-    
     ImGui::BeginChild("EntityList");
     
     for (EntityID id : SceneManager::Get().GetRootEntities()) {
@@ -45,6 +40,25 @@ void HierarchyPanel::Render() {
         }
     }
     
+    if (ImGui::BeginPopupContextWindow("HierarchyContextMenu", ImGuiPopupFlags_MouseButtonRight | ImGuiPopupFlags_NoOpenOverItems)) {
+        RenderCreateMenu(INVALID_ENTITY);
+        ImGui::EndPopup();
+    }
+    
+    ImGui::InvisibleButton("##DragTarget", ImVec2(-1, -1));
+    if (ImGui::BeginDragDropTarget()) {
+        if (const ImGuiPayload* payload = ImGui::AcceptDragDropPayload("ENTITY_ID")) {
+            EntityID sourceId = *(const EntityID*)payload->Data;
+            if (sourceId != INVALID_ENTITY) {
+                const Entity* sourceEntity = SceneManager::Get().GetEntity(sourceId);
+                if (sourceEntity && sourceEntity->parent != INVALID_ENTITY) {
+                    SceneManager::Get().MoveEntity(sourceId, INVALID_ENTITY);
+                }
+            }
+        }
+        ImGui::EndDragDropTarget();
+    }
+    
     ImGui::EndChild();
     
     ImGui::End();