refactor: 前端性能优化 — 倒计时组件隔离、React memoization 链路

- 新建 RefreshCountdown 组件，内部持有 timer，消除 App 每秒重渲染
- TargetBoard 分组逻辑 useMemo 化，避免 targets 引用不变时重复计算
- TargetGroup 加 React.memo，阻断无效渲染
- TrendChart 加 React.memo + chartData useMemo，避免 recharts 不必要重绘
- OverviewTab 统计项去掉 Card 包裹，改用纯 CSS 实现视觉效果
- 同步更新 refresh-control 和 target-detail-drawer spec

性能提升：消除每秒全组件树重渲染，减少 DOM 节点数

src/web/components/OverviewTab.tsx

@@ -1,6 +1,6 @@
 import type { ReactNode } from "react";
 
-import { Card, Col, Descriptions, Divider, Row, Skeleton, Space, Statistic, Typography } from "tdesign-react";
+import { Col, Descriptions, Divider, Row, Skeleton, Space, Statistic, Typography } from "tdesign-react";
 
 import type { TargetMetricsResponse, TargetStatus } from "../../shared/api";
 
@@ -97,11 +97,11 @@ export function OverviewTab({ metricsData, metricsLoading, target }: OverviewTab
 
 function OverviewStatItem({ color, suffix, title, value }: OverviewStatItemProps) {
   return (
-    <Card bordered={false} className="overview-stat-card" size="small">
+    <div className="overview-stat-card">
       <div className="overview-stat-item">
         <Typography.Text theme="secondary">{title}</Typography.Text>
         <Statistic className="overview-stat-value" color={color} suffix={suffix} value={value} />
       </div>
-    </Card>
+    </div>
   );
 }