Partially revert issue #224: restore hover-to-show close button behavior on desktop devices

[dpkass]

I’d like to propose partially reverting issue #224.

The previous behavior should be restored for desktop devices. On mobile it makes sense to keep the button always visible for accessibility and touch targets.

On desktop platforms, especially macOS, this is actually the expected interaction pattern (mirroring the native notification bar behavior), so users are familiar with it.

A reasonable approach would be:

@media (hover: hover) {
  [data-sonner-toast] [data-close-button] {
    opacity: 0;
    visibility: hidden;
  }
  [data-sonner-toast]:hover [data-close-button],
  [data-sonner-toast]:focus-within [data-close-button] {
    opacity: 1;
    visibility: visible;
  }
}

This would reintroduce hover-to-show behavior only for pointer/hover-capable devices, leaving mobile UX unchanged.