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/data.go

@@ -123,6 +123,7 @@ func (m *Model) refreshLive() {
 		ordered = filterSites(ordered, m.filterText)
 	}
 	m.sites = ordered
+	m.buildMaintSet()
 	m.refreshLogContent()
 
 	if m.selectedID != 0 {