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/update.go

@@ -142,7 +142,7 @@ func (m *Model) handleFormMsg(msg tea.Msg) (tea.Model, tea.Cmd) {
 	return m, nil
 }
 
-const detailInlineHeight = 10
+const detailInlineHeight = 11
 
 func (m *Model) recalcLayout() {
 	chrome := chromeBase