fix ($plugin-pwa): popup zindex conflict (fix #2642)

[smwbtech]

Summary
Changed z-index from 2 to 3 to avoid conflict with div[class*="language-"]::before elements

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• Bugfix
• Feature
• Code style update
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Other information:

[d-pollard]

LGTM

[bencodezen]

Thanks for the PR @smwbtech!