…esher behind content

The native iOS refresher (ion-refresher.refresher-native) has z-index: 1
while the inner scroll element had z-index: 0. This caused the refresher
spinner to remain visible on top of the scroll content when the user
scrolled back up after triggering a pull-to-refresh.

Increasing z-index from 0 to 2 ensures the scroll content renders above
the native refresher spinner. The z-index is still a non-auto integer
value so the WebKit stacking context optimization for scroll performance
(https://bugs.webkit.org/show_bug.cgi?id=216701) is preserved.

Fixes ionic-team#22358

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>