Migrate most of XWiki's LESS files to native CSS

Here is the order I'm following to convert the remaining LESS files, to make the progress easier to follow. The files are grouped in waves of increasing difficulty, and I keep at most 5 conversion PRs open at once so that reviews don't pile up.

Progress on 2026-10-19: 5/22 issues done, 2 PRs in review.

Wave A: trivial

Plain rules and simple variables, or dead code that can be removed.

#	Issue	File	Status
1	XWIKI-23665	custom.less	Done
2	XWIKI-23659	modals.less	Done
3	XWIKI-23662	figure.less	In review: #6213
4	XWIKI-23675	tree.less	Done
5	XWIKI-23678	headers.less	In review: #6899
6	XWIKI-23663	type.less	Done
7	XWIKI-23658	xlist.less	In progress
8	XWIKI-23666	reactive.less (livedata webjar)	Not started

Wave B: vars only

Uses LESS variables that need a CSS custom property counterpart.

#	Issue	File	Status
1	XWIKI-23673	vertical-menus.less	In progress
2	XWIKI-23668	panels.less	Not started
3	XWIKI-23672	drawer.less	Not started
4	XWIKI-23656	tree.less (tree webjar)	Not started

Wave C: mixins

Calls LESS mixins that have to be inlined or replaced.

Wave D: heavy

Large files that rely heavily on LESS features.

Wave E: last

Depends on the other waves being done.

#	Issue	File	Status
1	XWIKI-23664	variables.less	Done
2	XWIKI-23651	26 LESS SSX pages, split by module	Not started

Not triaged yet

These skin files have no issue yet: accordion.less, action-menus.less, annotations.less, before-bootstrap.less, breadcrumbs.less, buttons.less, livetable.less, messages.less.