fix: active sessions query, chart update performance

- Active sessions query now finds truly active sessions (started
  anytime, no session.end ever) instead of only today's sessions
- Use uPlot setData() for live WS updates instead of destroying
  and recreating the chart on every event
- Destroy chart only on window change so it recreates with new scale

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

cmd/web-ui/static/app.js

@@ -1137,6 +1137,11 @@
 
   async function loadTimeseries() {
     try {
+      // Destroy chart so it's recreated with new window scale
+      if (dashboardChart) {
+        dashboardChart.destroy();
+        dashboardChart = null;
+      }
       const data = await api('/v1/stats/timeseries?window=' + dashboardState.window);
       if (!isCurrentPath('/')) return;
       dashboardState.timeseries = data;
@@ -1146,28 +1151,35 @@
     }
   }
 
+  function buildChartData() {
+    const ts = dashboardState.timeseries;
+    if (!ts || !ts.series || ts.series.length === 0) return null;
+    return [
+      ts.series.map(b => Math.floor(new Date(b.ts).getTime() / 1000)),
+      ts.series.map(b => b.runs),
+      ts.series.map(b => b.tools),
+      ts.series.map(b => b.errors),
+    ];
+  }
+
   function renderTimeseriesChart() {
     const container = document.getElementById('dash-chart');
     if (!container || !dashboardState.timeseries) return;
 
-    const ts = dashboardState.timeseries;
-    if (!ts.series || ts.series.length === 0) {
+    const data = buildChartData();
+    if (!data) {
       container.innerHTML = '<p class="empty-state" style="padding:2rem">No data for this window</p>';
       return;
     }
 
+    // If chart already exists, just update the data
     if (dashboardChart) {
-      dashboardChart.destroy();
-      dashboardChart = null;
+      dashboardChart.setData(data);
+      return;
     }
 
     container.innerHTML = '';
 
-    const timestamps = ts.series.map(b => Math.floor(new Date(b.ts).getTime() / 1000));
-    const runs = ts.series.map(b => b.runs);
-    const tools = ts.series.map(b => b.tools);
-    const errors = ts.series.map(b => b.errors);
-
     const width = container.clientWidth || 600;
     const height = 200;
 
@@ -1217,7 +1229,7 @@
       ],
     };
 
-    dashboardChart = new uPlot(opts, [timestamps, runs, tools, errors], container);
+    dashboardChart = new uPlot(opts, data, container);
 
     if (dashboardResizeObserver) {
       dashboardResizeObserver.disconnect();