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/ViewportPanel.cpp

@@ -533,10 +533,10 @@ void ViewportPanel::RenderGlobalLeftToolbar(ImVec2 contentPos, ImVec2 /*contentS
 // ---------------------------------------------------------------------------
 // Render
 // ---------------------------------------------------------------------------
-void ViewportPanel::Render()
+void ViewportPanel::Render(bool* p_open)
 {
     ImGui::PushStyleVar(ImGuiStyleVar_WindowPadding, ImVec2(0, 0));
-    ImGui::Begin("Viewport", nullptr,
+    ImGui::Begin("Viewport", p_open,
                  ImGuiWindowFlags_NoCollapse      |
                  ImGuiWindowFlags_NoScrollbar     |
                  ImGuiWindowFlags_NoScrollWithMouse);