feat: 重构 Dashboard 为卡片式分组布局

表格布局替换为按分组展示的卡片式布局，新增 group 字段配置和 TargetBoard/TargetCard 等组件。模态框详情页支持时间范围筛选和分页，SummaryCards 减为 3 个。API 端点变更：trend/history 改用 from/to 参数，history 支持分页。recentSampleCount 硬编码为 30。

src/web/components/TargetGroup.tsx

@@ -0,0 +1,21 @@
+import type { TargetStatus } from "../../shared/api";
+import { GroupHeader } from "./GroupHeader";
+import { CardGrid } from "./CardGrid";
+
+interface TargetGroupProps {
+  name: string;
+  targets: TargetStatus[];
+  onTargetClick: (target: TargetStatus) => void;
+}
+
+export function TargetGroup({ name, targets, onTargetClick }: TargetGroupProps) {
+  const up = targets.filter((t) => t.latestCheck?.success && t.latestCheck?.matched).length;
+  const down = targets.length - up;
+
+  return (
+    <div className="target-group">
+      <GroupHeader name={name} total={targets.length} up={up} down={down} />
+      <CardGrid targets={targets} onTargetClick={onTargetClick} />
+    </div>
+  );
+}