feat: Dashboard 刷新频率可配置 — RadioGroup 选择器、动态轮询间隔、手动刷新按钮

- useDashboard hook 改为接受 refetchInterval 动态参数，移除固定 8 秒常量
- Header operations 区域重构为 RadioGroup（手动/10秒/30秒/1分钟/5分钟）+ 倒计时/刷新按钮
- 新增 formatCountdown 工具函数及单元测试
- 新增 .dashboard-refresh-control 和 .dashboard-countdown CSS 类
- 同步更新 DEVELOPMENT.md、README.md、主 specs

src/web/hooks/use-target-detail.ts

@@ -17,7 +17,7 @@ export function useTargetDetail() {
   const [timeTo, setTimeTo] = useState("");
   const [historyPage, setHistoryPage] = useState(1);
 
-  const { data: dashboardData } = useDashboard();
+  const { data: dashboardData } = useDashboard(false);
   const selectedTarget =
     selectedTargetId !== null
       ? (dashboardData?.targets.find((target) => target.id === selectedTargetId) ?? null)