Sticky <thead> bar of plug-ins-list

[gbdesign2023]

The bar of plug-ins-list table under "System" gets stuck when scrolling.

• Tested with iOS 17.3
• Chrome and Safari
• Kirby 4.1

https://github.com/getkirby/kirby/assets/140849227/f53aea2e-7a50-49e7-adf1-11e99a9cdbe9

[distantnative]

Comment for the team: Originates from the extra offset set to accommodate the sticky header. But the header here isn't sticky, so we need to ensure the extra offset is only applied when a sticky-able header is present.