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/RefreshCountdown.tsx

@@ -0,0 +1,52 @@
+import { useEffect, useState } from "react";
+import { RefreshIcon } from "tdesign-icons-react";
+import { Button, Typography } from "tdesign-react";
+
+import { formatCountdown } from "../utils/time";
+
+interface RefreshCountdownProps {
+  dashboardUpdatedAt: number;
+  isFetching: boolean;
+  isManualRefresh: boolean;
+  onRefresh: () => void;
+  refreshInterval: number;
+}
+
+export function RefreshCountdown({
+  dashboardUpdatedAt,
+  isFetching,
+  isManualRefresh,
+  onRefresh,
+  refreshInterval,
+}: RefreshCountdownProps) {
+  const [now, setNow] = useState(() => new Date());
+
+  useEffect(() => {
+    const timer = window.setInterval(() => setNow(new Date()), 1000);
+    return () => window.clearInterval(timer);
+  }, []);
+
+  const nextRefreshSeconds =
+    dashboardUpdatedAt > 0 && !isManualRefresh
+      ? Math.max(0, Math.ceil((dashboardUpdatedAt + refreshInterval - now.getTime()) / 1000))
+      : null;
+
+  if (isManualRefresh) {
+    return (
+      <Button
+        aria-label="刷新 Dashboard"
+        disabled={isFetching}
+        icon={<RefreshIcon />}
+        loading={isFetching}
+        onClick={() => void onRefresh()}
+        shape="circle"
+        variant="outline"
+      />
+    );
+  }
+
+  const refreshText =
+    dashboardUpdatedAt > 0 ? (isFetching ? "刷新中..." : formatCountdown(nextRefreshSeconds ?? 0)) : "等待首次刷新";
+
+  return <Typography.Text theme="secondary">{refreshText}</Typography.Text>;
+}