Use ::backdrop for Dialog Backgrounds

Use ::backdrop pseudo-element for dialog/popover backgrounds.

Incorrect (extra overlay node):

<>
  <div className={styles.overlay} onClick={close} />
  <dialog className={styles.dialog}>{children}</dialog>
</>

Correct (native ::backdrop):

dialog::backdrop {
  background: var(--black-a6);
  backdrop-filter: blur(4px);
}