Add Chart Builder: no-JSON Vega-Lite composer from a dataset (M4)

src/app/components/DatasetsModal.tsx

@@ -16,7 +16,7 @@ import { useState } from 'react';
 import { useShallow } from 'zustand/react/shallow';
 import { datasetReference, type DataSource, type Dataset } from '@core/dataset';
 import { detectFormat, detectFormatFromUrl, type DataFormat } from '@core/format-detection';
-import { closeModal, resnapshot } from '../modals/ModalCoordinator';
+import { closeModal, openModal, resnapshot } from '../modals/ModalCoordinator';
 import { confirm } from '../stores/ConfirmStore';
 import { notify } from '../stores/NotificationStore';
 import { selectSelectedDataset, useDatasetStore, byModifiedDesc } from '../stores/DatasetStore';
@@ -221,9 +221,16 @@ function DatasetDetail({
           <button type="button" className={styles.action} onClick={handleEdit}>
             Edit
           </button>
-          {/* "Build Chart from dataset" (spec §05) lands enabled with the Chart
-              Builder in M4. Per council (GOV.UK / NN/g), we don't ship a dead
-              disabled control in the meantime — the action appears when it works. */}
+          {/* Build Chart (spec §05 → §06) — opens the Chart Builder on this dataset.
+              Replaces the Datasets modal (one modal at a time, §01C); detail view has
+              no transient form state, so no discard prompt. */}
+          <button
+            type="button"
+            className={styles.action}
+            onClick={() => openModal('chartBuilder', String(dataset.id))}
+          >
+            Build Chart
+          </button>
           <button
             type="button"
             className={`${styles.action} ${styles.danger}`}