Consistent scrollbar styling across browsers

- colors.css: added --scrollbar-thumb, --scrollbar-thumb-hover, --scrollbar-track, --scrollbar-corner custom properties
- base.css: consolidated all scrollbar rules (WebKit ::-webkit-scrollbar* + Firefox scrollbar-width/color), added ::-webkit-scrollbar-corner, changed selector from * to html (scrollbar-width inherits), added scrollbar-gutter: stable
- admin.css: removed redundant scrollbar-width: thin on header nav
- toc.css: removed redundant scrollbar-width: thin on desktop .toc

app/public/assets/css/admin.css

@@ -14,7 +14,6 @@
 .admin-body header nav {
   overflow-x: auto;
   white-space: nowrap;
-  scrollbar-width: thin;
 }
 .admin-body header nav .nav-right-links {
   flex-shrink: 0;