feat(metrics): add global peak mark line to all live metric charts

Finds the series with the highest value across all datasets and adds
a SeriesMarkTypeMax dashed mark line to it. Since all series share the
same Y axis this effectively shows a single "global peak" line for the
whole chart with a label on the right.

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

audit/internal/webui/server.go

@@ -634,6 +634,12 @@ func renderChartSVG(title string, datasets [][]float64, names []string, labels [
 		}}
 	}
 
+	// Add a single peak mark line on the series that holds the global maximum.
+	peakIdx, peakVal := globalPeakSeries(datasets)
+	if peakIdx >= 0 && peakVal > 0 && peakIdx < len(opt.SeriesList) {
+		opt.SeriesList[peakIdx].MarkLine = gocharts.NewMarkLine(gocharts.SeriesMarkTypeMax)
+	}
+
 	p := gocharts.NewPainter(gocharts.PainterOptions{
 		OutputFormat: gocharts.ChartOutputSVG,
 		Width:        1400,
@@ -645,6 +651,21 @@ func renderChartSVG(title string, datasets [][]float64, names []string, labels [
 	return p.Bytes()
 }
 
+// globalPeakSeries returns the index of the series containing the global maximum
+// value across all datasets, and that maximum value.
+func globalPeakSeries(datasets [][]float64) (idx int, peak float64) {
+	idx = -1
+	for i, ds := range datasets {
+		for _, v := range ds {
+			if v > peak {
+				peak = v
+				idx = i
+			}
+		}
+	}
+	return idx, peak
+}
+
 func sanitizeChartText(s string) string {
 	if s == "" {
 		return ""