Add per-viewport render settings persistence (delegate, grid, AA, bg, bbox, lighting)

ViewportTileSettings struct captures: render delegate, show grid, AA, background
color, bbox mode, ambient-light-only, dome-light flags.

ViewportPanel::SaveSettings() writes a simple INI file (layout + one [TileN]
section per tile) to %APPDATA%\UsdLayerManager\viewport_settings.ini on
Application::Shutdown().

ViewportPanel::LoadSettings() is called at the end of Application::Initialize()
— after the viewport panel is constructed but before the first frame — so the
saved render delegate is picked up by UsdSceneRenderer::InitRenderer() on the
first Render() call.  Layout (Single/HSplit/VSplit/Quad) and split ratios are
also restored, creating the correct number of tiles before settings are applied.

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

src/ui/Application.cpp

@@ -138,6 +138,15 @@ bool Application::Initialize(const std::string& windowTitle, int width, int heig
         RefreshManagers();
     }
     
+    // Load per-viewport settings (render delegate, grid, AA, etc.) from AppData.
+    if (const char* appData = getenv("APPDATA")) {
+        namespace fs = std::filesystem;
+        fs::path dir = fs::path(appData) / "UsdLayerManager";
+        fs::create_directories(dir);
+        m_viewportSettingsPath = (dir / "viewport_settings.ini").string();
+        m_viewportPanel->LoadSettings(m_viewportSettingsPath);
+    }
+
     LOG_INFO("Application initialized successfully");
     return true;
 }
@@ -155,6 +164,9 @@ void Application::Run() {
 }
 
 void Application::Shutdown() {
+    if (m_viewportPanel && !m_viewportSettingsPath.empty())
+        m_viewportPanel->SaveSettings(m_viewportSettingsPath);
+
     m_viewportPanel.reset();
     m_sceneHierarchyPanel.reset();
     m_propertyPanel.reset();