Skore: slim scrollbar v style hry namiesto OS defaultu

Zoznam kol v Standings (herny sidebar aj mobilny panel) pouzival hrubu
systemovu lavicu scrollbaru, ktora nesedela do velvet/gold temy. Novy
.velvet-scroll (index.css) da tenky priehladny track a zlaty polopriehladny
thumb (webkit + Firefox scrollbar-color).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/components/Standings.tsx

@@ -148,7 +148,7 @@ export default function Standings({ standings, guesses = [], players, myOrder, d
   });
 
   const scrollableRounds = (
-    <div className={`flex-1 min-h-0 overflow-y-auto px-3 ${desktop ? '' : 'max-h-[45vh]'}`}>
+    <div className={`velvet-scroll flex-1 min-h-0 overflow-y-auto px-3 ${desktop ? '' : 'max-h-[45vh]'}`}>
       {rows}
 
       {/* Active round placeholder */}