chore: add TUI screenshots via VHS

Screenshots capture 4 views: monitors dashboard (hero), detail panel,
alerts tab, and logs tab. Includes VHS tape, demo seed config, and
setup script for reproducible captures.

Also fixes latencySparkline to color DOWN checks red instead of green
— previously failed checks with 0ms latency rendered as green bars.

internal/tui/tab_sites.go

@@ -60,14 +60,18 @@ type siteFormData struct {
 	Regions       string
 }
 
-func latencySparkline(latencies []time.Duration, width int) string {
+func latencySparkline(latencies []time.Duration, statuses []bool, width int) string {
 	if len(latencies) == 0 {
 		return subtleStyle.Render(strings.Repeat("·", width))
 	}
 
 	samples := latencies
+	sampledStatuses := statuses
 	if len(samples) > width {
 		samples = samples[len(samples)-width:]
+		if len(sampledStatuses) > width {
+			sampledStatuses = sampledStatuses[len(sampledStatuses)-width:]
+		}
 	}
 
 	minL, maxL := samples[0], samples[0]
@@ -85,7 +89,7 @@ func latencySparkline(latencies []time.Duration, width int) string {
 		sb.WriteString(subtleStyle.Render(strings.Repeat("·", remaining)))
 	}
 	spread := maxL - minL
-	for _, l := range samples {
+	for i, l := range samples {
 		idx := 0
 		if spread > 0 {
 			idx = int(float64(l-minL) / float64(spread) * 7)
@@ -94,13 +98,18 @@ func latencySparkline(latencies []time.Duration, width int) string {
 			}
 		}
 		ch := string(sparkChars[idx])
-		ms := l.Milliseconds()
-		if ms < 200 {
-			sb.WriteString(specialStyle.Render(ch))
-		} else if ms < 500 {
-			sb.WriteString(warnStyle.Render(ch))
-		} else {
+		isDown := i < len(sampledStatuses) && !sampledStatuses[i]
+		if isDown {
 			sb.WriteString(dangerStyle.Render(ch))
+		} else {
+			ms := l.Milliseconds()
+			if ms < 200 {
+				sb.WriteString(specialStyle.Render(ch))
+			} else if ms < 500 {
+				sb.WriteString(warnStyle.Render(ch))
+			} else {
+				sb.WriteString(dangerStyle.Render(ch))
+			}
 		}
 	}
 	return sb.String()
@@ -474,7 +483,7 @@ func (m Model) viewSitesTab() string {
 				if site.Type == "push" {
 					spark = heartbeatSparkline(hist.Statuses, sparkWidth)
 				} else {
-					spark = latencySparkline(hist.Latencies, sparkWidth)
+					spark = latencySparkline(hist.Latencies, hist.Statuses, sparkWidth)
 				}
 
 				rows = append(rows, []string{
@@ -949,7 +958,7 @@ func (m Model) viewDetailPanel() string {
 				up, len(hist.Statuses))
 		}
 	} else {
-		b.WriteString("  " + latencySparkline(hist.Latencies, sparkWidth))
+		b.WriteString("  " + latencySparkline(hist.Latencies, hist.Statuses, sparkWidth))
 		if len(hist.Latencies) > 0 {
 			minL, maxL := hist.Latencies[0], hist.Latencies[0]
 			var total time.Duration