Scene Hierarchy ET switcher + Create Sublayer via Save dialog

- Add edit-target layer combo at top of Scene Hierarchy panel (always
  visible when stage loaded — session, root, sublayers); routes through
  LayerManager::SetEditTarget so StageEditor ET checkboxes stay in sync
- Replace custom Create Sublayer modal with native SaveFile dialog;
  LayerManager::CreateNewSublayer creates the file on disk, inserts it
  at the top of the sublayer stack, and activates it as edit target

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/ui/Application.cpp

@@ -69,6 +69,7 @@ bool Application::Initialize(const std::string& windowTitle, int width, int heig
     m_sceneHierarchyPanel = std::make_unique<SceneHierarchyPanel>();
     m_sceneHierarchyPanel->SetPropertyManager(m_propertyManager.get());
     m_sceneHierarchyPanel->SetCommandHistory(&m_commandHistory);
+    m_sceneHierarchyPanel->SetLayerManager(m_layerManager.get());
     m_viewportPanel = std::make_unique<ViewportPanel>();
     m_viewportPanel->SetCommandHistory(&m_commandHistory);
     m_propertyPanel = std::make_unique<PropertyPanel>();