View menu — panel visibility toggles for all 5 panels

Each panel (Stage Editor, Scene Hierarchy, Viewport, Property Panel,
Timeline) now has a checkable menu item under View. Closing a panel via
its X button also unchecks the entry. Stage Info and Demo Window follow
below a separator.

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

src/ui/Application.h

@@ -29,6 +29,7 @@ private:
     void Update();
     void RenderUI();
     void RenderMenuBar();
+    void RenderStatusBar();
     void RenderStageInfo();
     void RefreshManagers();
     
@@ -56,6 +57,11 @@ private:
     std::unique_ptr<TimelinePanel> m_timelinePanel;
     bool m_showDemoWindow;
     bool m_showStageInfo;
+    bool m_showStageEditor;
+    bool m_showSceneHierarchy;
+    bool m_showViewport;
+    bool m_showPropertyPanel;
+    bool m_showTimeline;
     bool m_running;
 };