Fixed the mobile menu button's animation to match the original exactly

Prism's rebuilt mobile menu button was meant to reproduce the previous site's hamburger-to-X animation exactly, and looked right at a glance, but a direct side-by-side comparison turned up two small, real differences: the animation used a different easing curve than the one it was meant to match (same name, different math — a common mix-up between a design-system utility and the literal CSS keyword it's named after), and the bars themselves were about 9% wider than the original icon.

Both are corrected now, matching the original hamburger icon's exact geometry and motion.