feat(tui): overhaul latency sparkline scaling, color, and layout

Replace misleading relative-only sparkline with dual-channel design:
bar height uses relative scaling (shows stability and anomalies),
color+brightness uses absolute thresholds (shows fast vs slow).

- Add brightness gradient within color bands (dim→bright as latency
  increases toward the next threshold)
- Pass row background through sparkline rendering so zebra stripes
  and selection highlights carry through ANSI sequences
- Cap sparkline width to 60 (matches maxHistoryLen) and column
  width to 62 to eliminate trailing dead space
- Quiet group sparkline: subtle dots for healthy, bold red for down
- Add braille subpixel canvas (ported from meridian) for future
  multi-row graph use

internal/tui/view_detail.go

@@ -203,7 +203,7 @@ func (m Model) viewDetailPanel() string {
 	b.WriteString(m.divider() + "\n")
 	const sparkWidth = 40
 	if site.Type == "push" {
-		b.WriteString("  " + heartbeatSparkline(hist.Statuses, sparkWidth))
+		b.WriteString("  " + heartbeatSparkline(hist.Statuses, sparkWidth, ""))
 		if len(hist.Statuses) > 0 {
 			up := 0
 			for _, s := range hist.Statuses {
@@ -216,7 +216,7 @@ func (m Model) viewDetailPanel() string {
 				up, len(hist.Statuses))
 		}
 	} else {
-		b.WriteString("  " + latencySparkline(hist.Latencies, hist.Statuses, sparkWidth))
+		b.WriteString("  " + latencySparkline(hist.Latencies, hist.Statuses, sparkWidth, ""))
 		var minL, maxL, total time.Duration
 		count := 0
 		for i, l := range hist.Latencies {