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.h

@@ -50,7 +50,7 @@ public:
     void SetSelectedPrimPath(const std::string& path);
 
     // ── Main render (called from Application::RenderUI) ──────────────────────
-    void Render();
+    void Render(bool* p_open = nullptr);
 
     // ── Pick callbacks (wired by Application after construction) ─────────────
     std::function<void(const std::string&)>              OnPrimPicked;