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/TargetGroup.tsx

@@ -1,5 +1,6 @@
 import type { PrimaryTableCol } from "tdesign-react";
 
+import { memo } from "react";
 import { Card, PrimaryTable, Space, Tag } from "tdesign-react";
 
 import type { TargetStatus } from "../../shared/api";
@@ -11,7 +12,7 @@ interface TargetGroupProps {
   targets: TargetStatus[];
 }
 
-export function TargetGroup({ columns, name, onTargetClick, targets }: TargetGroupProps) {
+export const TargetGroup = memo(function TargetGroup({ columns, name, onTargetClick, targets }: TargetGroupProps) {
   const up = targets.filter((t) => t.latestCheck?.matched).length;
   const down = targets.length - up;
   const displayName = name === "default" ? "默认分组" : name;
@@ -48,4 +49,4 @@ export function TargetGroup({ columns, name, onTargetClick, targets }: TargetGro
       />
     </Card>
   );
-}
+});