feat(store): detect overlapping maintenance windows

Warn via log when creating a maintenance window that overlaps with an
existing one for the same monitor, parent group, or global scope.
Handles both timed and indefinite windows.

internal/tui/tab_maint.go

@@ -256,6 +256,11 @@ func (m *Model) submitMaintForm() tea.Cmd {
 	st := m.store
 	m.state = stateDashboard
 	return writeCmd("Add maintenance window", func() error {
-		return st.AddMaintenanceWindow(context.Background(), mw)
+		ctx := context.Background()
+		overlaps, _ := st.GetOverlappingMaintenanceWindows(ctx, mw.MonitorID, mw.StartTime, mw.EndTime)
+		if len(overlaps) > 0 {
+			_ = st.SaveLog(ctx, fmt.Sprintf("Overlap: new window %q overlaps with existing %q", mw.Title, overlaps[0].Title))
+		}
+		return st.AddMaintenanceWindow(ctx, mw)
 	})
 }