feat(tui): add SLA reporting view

Full-screen SLA report accessible via [s] from detail panel.
Computes uptime%, downtime, outage count, longest outage, MTTR,
and MTBF from state_changes table. Includes daily breakdown with
bar chart, switchable time periods (24h/7d/30d/90d), and
scrollable viewport. LATE/STALE treated as UP for SLA purposes.

internal/tui/view_dashboard.go

@@ -98,6 +98,8 @@ func (m Model) View() string {
 		return m.viewDetailPanel()
 	case stateHistory:
 		return m.viewHistoryPanel()
+	case stateSLA:
+		return m.viewSLAPanel()
 	case stateAlertDetail:
 		return m.viewAlertDetailPanel()
 	default: