ngui: yield CPU during standby mode

when the screen turns off, the idle loop never gave a chance CPU to
do other work and pegged at 100%. adding a short sleep each cycle
yields CPU to the OS.

closes #15

src/ui/screen.zig

@@ -30,6 +30,7 @@ pub fn sleep(wake: *const Thread.ResetEvent) void {
             return;
         }
         std.atomic.spinLoopHint();
+        std.time.sleep(10 * std.time.ns_per_ms);
     }
 }