fix: replace usleep with nanosleep for POSIX compliance

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

gui.c

@@ -5,6 +5,7 @@
 #include <jack/jack.h>
 #include <ncurses.h>
 #include <unistd.h>
+#include <time.h>
 #include "engine.h"
 #include "gui.h"
 
@@ -200,7 +201,8 @@ int gui_main(Engine *engine)
         }
 
         refresh();
-        usleep(16666); /* ~60 fps */
+        struct timespec ts = {0, 16666000}; /* 16666 us = 16666000 ns */
+        nanosleep(&ts, NULL);
     }
 
     /* cleanup */