Route UI state save through platform services

src/platform_api/platform_services.h

@@ -46,6 +46,7 @@ public:
     [[nodiscard]] virtual std::vector<std::string> document_browse_roots(
         std::string_view work_path,
         std::string_view data_path) = 0;
+    virtual void save_ui_state() = 0;
     [[nodiscard]] virtual bool enables_live_asset_reloading() = 0;
     virtual void update_platform_frame(float delta_time_seconds) = 0;
     virtual void report_rendered_frames(int frames) = 0;