feat(tui): add state change history view with outage duration

Full-screen scrollable history view accessible via [h] from detail
panel. Shows all state transitions with computed outage durations,
event density sparkline for flapping detection, and summary stats.

- Detail panel STATE CHANGES now shows outage duration per recovery
- Event density sparkline highlights flapping periods
- Summary footer: event count, outage count, avg outage duration
- Vim-style navigation (j/k/g/G) + mouse scroll in history view

internal/tui/view_dashboard.go

@@ -96,6 +96,8 @@ func (m Model) View() string {
 		return ""
 	case stateDetail:
 		return m.viewDetailPanel()
+	case stateHistory:
+		return m.viewHistoryPanel()
 	case stateAlertDetail:
 		return m.viewAlertDetailPanel()
 	default: