fix: correct JACK shutdown callback signature and suppress unused parameter warning

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

main.c

@@ -8,6 +8,7 @@ static Engine engine;
 static volatile int keep_running = 1;
 
 void signal_handler(int sig) {
+    (void)sig;
     keep_running = 0;
 }