read willBeHidden from hiddenLabels in handleLegendClick

Address Greptile P2 review on PR #4259. willBeHidden was derived from
!ds.hidden, but ds.hidden can be set by the focusLabel pass too, so
the dataset flag is not a faithful read of the controlled hidden
state. In a chart that uses both focusLabel and hiddenLabels at once,
the callback could fire with a misleading toggle direction.

Read directly from hiddenLabels instead, with optional chaining so
the new path is a no-op when hiddenLabels is not provided. Behavior
is unchanged for EventsChart (which does not use focusLabel today)
but the controlled state is now the single source of truth.

src/components/charts/Chart.tsx

@@ -70,7 +70,7 @@ export function Chart({
       // and let the parent push a new hiddenLabels set on the next render.
       const { datasetIndex } = item;
       const ds = chart.current.data.datasets[datasetIndex];
-      onLegendClick(ds.label, !ds.hidden);
+      onLegendClick(ds.label, !hiddenLabels?.has(ds.label));
       return;
     }