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/base.css

@@ -11,30 +11,22 @@ body {
   overflow: hidden;
 }
 
-/* ── Discreet scrollbars ─────────────────────────────────────────── */
-/* WebKit browsers */
+/* ── Discreet scrollbars (WebKit / Blink / Firefox) ──────────────── */
+/* CSS custom properties defined in colors.css:
+   --scrollbar-thumb, --scrollbar-thumb-hover, --scrollbar-track, --scrollbar-corner */
 ::-webkit-scrollbar {
-  width: 5px;
-  height: 5px;
+  width:  var(--scrollbar-width, 5px);
+  height: var(--scrollbar-width, 5px);
 }
+::-webkit-scrollbar-track        { background: var(--scrollbar-track); }
+::-webkit-scrollbar-thumb        { background: var(--scrollbar-thumb); border-radius: 3px; }
+::-webkit-scrollbar-thumb:hover  { background: var(--scrollbar-thumb-hover); }
+::-webkit-scrollbar-corner       { background: var(--scrollbar-corner); }
 
-::-webkit-scrollbar-track {
-  background: transparent;
-}
-
-::-webkit-scrollbar-thumb {
-  background: var(--text-tertiary);
-  border-radius: 3px;
-}
-
-::-webkit-scrollbar-thumb:hover {
-  background: var(--text-secondary);
-}
-
-/* Firefox */
-* {
+html {
   scrollbar-width: thin;
-  scrollbar-color: var(--text-tertiary) transparent;
+  scrollbar-color: var(--scrollbar-thumb) var(--scrollbar-track);
+  scrollbar-gutter: stable;
 }
 
 body {