Create a metallic subscription button inspired by the Ultramock button snippet:
- Real button semantics with a copy-paste friendly React component.
- Chrome gradient surface using the live site's 135deg metallic color stops.
- Squircle-xl shape with superellipse corner-shape support where available.
- A pointer-driven chrome sheen layer using --mx so the highlight follows mouse movement.
- Small sparkle highlights, tactile hover/active states, focus-visible ring, disabled state, and reduced-motion handling.