style(frontend): refactor StatCard for DESIGN.md compliance

- Replaced 'hover:bg-surface-bright' with 'hover:bg-black/40'

frontend/components/StatCard.tsx

@@ -9,7 +9,7 @@ interface StatCardProps {
 
 export default function StatCard({ label, value, icon: Icon }: StatCardProps) {
   return (
-    <div className="flex justify-between items-center gap-2 p-2 md:p-3 bg-surface-container border border-border transition-all hover:bg-surface-bright" role="status">
+    <div className="flex justify-between items-center gap-2 p-2 md:p-3 level-1 transition-all hover:bg-black/40" role="status">
       <div className="flex items-center gap-2.5 min-w-0">
         {Icon && <Icon className="w-5 h-5 text-primary flex-shrink-0" aria-hidden="true" />}
         <span className="text-base md:text-lg text-secondary font-normal truncate">