fix: single instance - present existing window instead of creating new

linux/my_application.cc

@@ -20,6 +20,14 @@ G_DEFINE_TYPE(MyApplication, my_application, GTK_TYPE_APPLICATION)
 static void my_application_activate(GApplication *application)
 {
   MyApplication *self = MY_APPLICATION(application);
+  
+  // Check if window already exists - if so, just present it
+  GList* windows = gtk_application_get_windows(GTK_APPLICATION(application));
+  if (windows) {
+    gtk_window_present(GTK_WINDOW(windows->data));
+    return;
+  }
+  
   GtkWindow *window =
       GTK_WINDOW(gtk_application_window_new(GTK_APPLICATION(application)));
   gtk_window_set_icon_from_file(window, ICON_PATH, NULL);