fix: update GUI to use Engine struct and remove duplicate definitions

Co-authored-by: aider (deepseek/deepseek-coder) <aider@aider.chat>

test_gui.c

@@ -45,7 +45,7 @@ int main(void)
     int ret = gui_main(client, buffer, buffer_size);
 
     /* cleanup */
-    engine_cleanup(client);
+    engine_cleanup(g_engine);
     free(buffer);
     jack_client_close(client);