Revert overflow: overlay (revert #21850) (#25231) (#25239)

Backport #25231 by @wxiaoguang

It causes not only one issue like #25221 (the footer width was also
affected by that change and was fixed some time ago)

The problem of "overflow: overlay" (#21850) is:

* It's not widely supported and is non-standard
https://caniuse.com/css-overflow-overlay
* It's not widely tested in Gitea (some standard layout like `ui
container + ui grid` may break it).
* The benefit seems smaller than the problems it brings.

So, I think it is good to revert it.

----

Let's leave enough time for testing and reviewing.

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: silverwind <me@silverwind.io>

web_src/fomantic/_site/globals/site.variables

@@ -8,3 +8,4 @@
 @variationPopupTooltip: false;
 @linkHoverUnderline: underline;
 @variationButtonSocial: false;
+@pageOverflowX: visible;