Make DatasetStore the collision-free id authority

src/app/components/ChartBuilderModal.test.tsx

@@ -53,7 +53,9 @@ describe('ChartBuilderModal', () => {
       now: T,
     });
     useDatasetStore.getState().add(ds);
-    useChartBuilderStore.getState().init(ds.id);
+    // `add` reassigns a collision-free id (it is the id authority), so read it back.
+    const id = useDatasetStore.getState().datasets[0].id;
+    useChartBuilderStore.getState().init(id);
     useChartBuilderStore.getState().setMark('bar');
     expect(useChartBuilderStore.getState().config.mark).toBe('bar');