perf(tui): precompute maintenance set, eliminate redundant GetAllSites

Replace O(windows × sites) isMonitorInMaintenance with O(1) map lookup.
buildMaintSet runs once per refreshLive/handleTabData, not per call site.

groupSparkline/groupUptime now use m.sites (already on model) instead of
calling engine.GetAllSites() which copies the full map under a mutex.

internal/tui/update.go

@@ -231,6 +231,7 @@ func (m *Model) handleTabData(msg tabDataMsg) (tea.Model, tea.Cmd) {
 	}
 	m.nodes = msg.nodes
 	m.maintenanceWindows = msg.maint
+	m.buildMaintSet()
 	m.clampCursor()
 	return m, nil
 }