fix(ui): replace hardcoded chart hex colors with Warm Precision tokens

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/monitoring/Sparkline.tsx

@@ -7,7 +7,7 @@ interface SparklineProps {
   height?: number
 }
 
-export function Sparkline({ data, color = '#38BDF8', width = 60, height = 24 }: SparklineProps) {
+export function Sparkline({ data, color = 'hsl(var(--accent))', width = 60, height = 24 }: SparklineProps) {
   const chartData = data.map((v, i) => ({ v, i }))
   return (
     <LineChart width={width} height={height} data={chartData}>