Make DatasetStore the collision-free id authority

src/app/stores/ChartBuilderStore.test.ts

@@ -11,7 +11,9 @@ const T = new Date('2026-06-01T00:00:00Z');
 function seedDataset(name: string, data: unknown): number {
   const ds = createDataset({ name, data, format: 'json', source: 'inline', now: T });
   useDatasetStore.getState().add(ds);
-  return ds.id;
+  // `add` is the id authority and reassigns a collision-free id, so read it back
+  // rather than trusting the pre-add createDataset default.
+  return useDatasetStore.getState().datasets[0].id;
 }
 
 beforeEach(() => {