feat(tui): multi-row color-coded sparkline chart with Y-axis

Replace ntcharts with custom multi-row sparkline. Each bar is color-
coded by latency threshold (green < 200ms, yellow < 500ms, red > 500ms)
and DOWN checks render in red. 3 rows tall with 24 discrete levels.

Y-axis labels (max/min ms) on the left, Min/Avg/Max stats below.
Zero external dependencies — removed ntcharts.

internal/tui/view_detail_inline.go

@@ -71,7 +71,7 @@ func (m Model) viewDetailInline(width int) string {
 		if chartW < 20 {
 			chartW = 20
 		}
-		chart := m.latencyChart(hist.Latencies, hist.Statuses, chartW, 2)
+		chart := m.latencyChart(hist.Latencies, hist.Statuses, chartW, 3)
 		if chart != "" {
 			b.WriteString(chart + "\n")
 		}