Issue #3557364: Move mobile detection from PHP to JS to avoid cache issues

wouters_frequested to merge
3557364-move-mobile-detection-from-php-to-js into 4.0.x

Closes #3557364

Moves the prevent-auto-open-on-mobile logic from PHP (user-agent detection) to JavaScript (window.matchMedia). This eliminates per-visitor cache variations in Drupal page cache and Varnish.

Applied the patch from the issue, resolved a minor conflict in the event listener block.

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