[feat] add in-app updates for desktop installations

apps/desktop/src/gui/application/gui_application.h

@@ -22,6 +22,7 @@
 namespace crossdesk {
 
 class UpdateChecker;
+namespace updater { class Controller; }
 
 // Slint application shell. GuiRuntime remains the owner of transport, media,
 // device, clipboard, transfer and settings behavior.
@@ -126,6 +127,9 @@ private:
 
   std::unique_ptr<SlintUi> ui_;
   std::unique_ptr<UpdateChecker> update_checker_;
+#if CROSSDESK_HAS_UPDATER
+  std::unique_ptr<updater::Controller> updater_;
+#endif
   CursorStateProvider cursor_state_provider_;
   std::unordered_map<std::string, CursorDeliveryState>
       cursor_delivery_states_;