fix: move CarlaHost to Engine struct and fix TUI plugin dialog access

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

engine.c

@@ -191,6 +191,10 @@ int engine_init(Engine *engine, const char *client_name, DispatchFn dispatch) {
     
     engine->sample_rate = jack_get_sample_rate(engine->client);
     
+    // Initialize Carla host
+    carla_init(&engine->carla_host, engine->client);
+    carla_scan_plugins(&engine->carla_host);
+    
     return 0;
 }