fix: set scrollbar-gutter stable

[bluwy]

Description

Use scrollbar-gutter to always preserve a space for scrollbars, even if it exists or not. This addresses the layout shift when navigating between pages that have scrollbars or not, and when opening/closing the search modal

Linked Issues

fixes #4884
fixes #1054
alternative to #1844 (that PR makes the scrollbar overlay instead of taking up space) (if this is merged, closes #1844)

Additional Context

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Tip

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[brc-dd]

One issue with scrollbar-gutter is that the browser doesn't paint things behind the gutter. So, the navbar/footer/gradient will appear to be cut near the right when the gutter is empty. Hold off merging this for now, I'll take a look to see if there is some better way.

[bluwy]

In most cases it's not really obvious or visible, so I think it's fine. The worst case seems to be for Safari which forces an ugly theme-color background for the scrollbar (can't seem to workaround this):

I think it's still fine for Safari though, for those who use it. Otherwise the only other option is with #1844 which uses width: 100vw and overflow-x: hidden, which we might need to take special care of handling in a few places. Too bad browsers don't have a supported way to support gutters without taking up space like overflow-y: overlay.