Add CSS cascade layers for Compound, shared components, and app/web styles (#33302)

* Layer Compound and shared component CSS

* Layer app theme CSS

* Remove !important flags from ActionBarView

* Remove unnecessary !important statements from shared components

* Avoid dead code errors for *.pcss just because layer is specified after @import url

* Remove unnecessary !important styling

* Override Banner defaults in RoomStatusBarView

* Updated snaps

* Updated snaps

* Fix styling of media body in app/web

* Fix styling for Compound anchors

* Fix styling issues in app/web

* More styling fixes

* Fix a problem extracting css for HTMLExport

* Revert changes

* Fix for theme styling

* Add test to improve coverage

* Prettier

* Fix styling issues

* Add data-kind attribute to avoid global styling override

* Update screenshot that now is correct

* Revert data-kind attribute

* Handle LinkPreview styling in .pcss

* Fix flaky test: Avoid racing the lazy-loaded ManageEventIndexDialog

* Take care of review comments

* Updated snaps

* Updated snaps again after merge

* Remove !important from RoomStatusBar

apps/web/res/css/_common.pcss

@@ -163,9 +163,10 @@ b {
     font-weight: bold;
 }
 
-a:hover,
-a:link,
-a:visited {
+/* Keep the legacy link colour without overriding Compound anchors. */
+a:where(:not([data-kind])):hover,
+a:where(:not([data-kind])):link,
+a:where(:not([data-kind])):visited {
     color: $accent-alt;
 }