feat(tui): two-column detail layout with viewport scrolling

Refactored full-screen detail panel: status+endpoint on left column,
timing+config on right column. Halves vertical height of the info
section. Content wraps in a viewport when it exceeds terminal height
— scroll with j/k/arrows/pgup/pgdn.

Restored renderSparkTooltip lost in rewrite. Fixed lint warnings.

internal/tui/tui.go

@@ -200,6 +200,7 @@ type Model struct {
 	detailChanges       []models.StateChange
 	detailChangesSiteID int
 	detailDailyDays     []monitor.DayReport
+	detailViewport      viewport.Model
 
 	filterMode bool
 	filterText string