arena.mjs: fix cursor logic - show yellow cursor when unlocked
The pen lock state logic was inverted. Changed condition from 'if (penLocked)'
to 'if (!penLocked)' so the yellow crosshair appears when the pen is actually
unlocked/in FPS mode, and the default cyan cursor shows when locked.
Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>