fix: Gateway trend bars missing height — now render 48 visible bars

Bars had no `height` property, making them invisible despite the legend
showing correctly. Each bar now computes height proportional to anomaly
count (max 36px), with a 3px minimum for normal intervals.

Co-Authored-By: Claude <noreply@anthropic.com>

frontend/src/pages/DashboardPage.tsx

@@ -171,6 +171,10 @@ function barColor(item: GatewayTrendItem): string {
 }
 
 function GatewayTimeline({ trend }: { trend: GatewayTrendItem[] }) {
+  const maxIssues = Math.max(1, ...trend.map(
+    i => i.heartbeat_timeouts + i.feishu_disconnects + i.restarts,
+  ))
+
   return (
     <div>
       <div
@@ -184,6 +188,9 @@ function GatewayTimeline({ trend }: { trend: GatewayTrendItem[] }) {
       >
         {trend.map((item, i) => {
           const col = barColor(item)
+          const total = item.heartbeat_timeouts + item.feishu_disconnects + item.restarts
+          const pct = total / maxIssues
+          const height = total === 0 ? 3 : Math.max(3, Math.round(pct * 36))
           const details: string[] = []
           if (item.heartbeat_timeouts > 0) details.push('心跳超时 ' + item.heartbeat_timeouts)
           if (item.feishu_disconnects > 0) details.push('飞书断连 ' + item.feishu_disconnects)
@@ -195,6 +202,7 @@ function GatewayTimeline({ trend }: { trend: GatewayTrendItem[] }) {
               title={item.hour + ': ' + tip}
               style={{
                 flex: 1,
+                height,
                 borderRadius: '2px 2px 0 0',
                 minWidth: 3,
                 cursor: 'pointer',