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

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

engine.h

@@ -5,6 +5,7 @@
 #include <jack/midiport.h>
 #include <stdbool.h>
 #include "dispatcher.h"
+#include "carla.h"
 
 typedef struct {
     jack_client_t *client;
@@ -18,6 +19,8 @@ typedef struct {
     DispatchFn dispatch;
     jack_nframes_t sample_rate;
     bool running;
+    
+    CarlaHost carla_host;  // Carla host state for plugin management
 } Engine;
 
 int engine_init(Engine *engine, const char *client_name, DispatchFn dispatch);