fix(site): clip 3D tilt glare to card radius for Safari

[niklonskyi]

Summary

• Safari doesn't apply a card's overflow: hidden + border-radius clip to
  a composited child (mix-blend-mode: screen) when the card also has
  transform-style: preserve-3d. On the 3D Tilt card (P19), this let the
  cursor-tracked glare and static sheen paint past the rounded corners on
  hover, showing a flash of square 90° edges (right upper corner on the screenshot).
• Fix by giving .p19-glare and .p19-card::before their own
  border-radius: inherit, so they clip to the card's rounded corners.
• Applied to the live showcase and prototypes playground CSS, the
  copyable .t-tilt-glare template, and the skill payload