Route VR lifecycle through platform services

src/app.h

@@ -198,6 +198,8 @@ public:
     void display_file(std::string path);
     void share_file(std::string path);
     void request_app_close();
+    [[nodiscard]] bool start_platform_vr_mode();
+    void stop_platform_vr_mode();
     void attach_ui_thread();
     void detach_ui_thread();
     void acquire_render_context();