fix(dashboard): 修复资源趋势图取数变量未定义导致构建失败

e0cc547c 移除 generateMetricsData 时删掉了 `const data = ...`，
但 AreaChart 仍引用 `data`，触发 TS2304 "Cannot find name 'data'"，
前端 tsc -b 无法通过。改为引用实际的 realData。

// Logan.Li

frontend/src/components/dashboard/DashboardCharts.tsx

@@ -344,7 +344,7 @@ export function ResourceUsageCard({
         {realData.length > 0 ? (
         <div className="h-[280px]">
           <ResponsiveContainer width="100%" height="100%">
-            <AreaChart data={data} margin={{ top: 10, right: 10, left: 0, bottom: 0 }}>
+            <AreaChart data={realData} margin={{ top: 10, right: 10, left: 0, bottom: 0 }}>
               <defs>
                 <linearGradient id="colorCpu" x1="0" y1="0" x2="0" y2="1">
                   <stop offset="5%" stopColor="var(--dg-blue)" stopOpacity={0.3} />