fix(workflow): re-fetch dataset details when workflow nodes change

## DatasetsDetailProvider fetches dataset metadata only on mount, leaving store stale when workflow graph changes

`web/app/components/workflow/datasets-detail-store/provider.tsx` lines 34-46 — the effect that fetches dataset details for knowledge-retrieval nodes has an empty dependency array, but reads the `nodes` prop. As a result, the fetch only happens when the provider first mounts. Any subsequent change to the workflow graph leaves the dataset-detail store in its initial state, even though the graph on screen has been replaced.

### Trigger scenarios

- Loading a different saved workflow while the workflow editor stays mounted.
- Real-time collaboration: another user adds, removes, or changes the dataset IDs of a knowledge-retrieval node.
- Undo / redo of node additions or deletions.
- Adding or removing a knowledge-retrieval node after the initial mount.
- Snippet insertion that introduces or rewires nodes.

In each case, consumers of the store (dataset pickers, dataset-name previews, validation messages that need the dataset title) display either the original dataset list, missing entries, or datasets that no longer exist on the workflow.

### Expected

The dataset-detail store reflects the current set of datasets referenced by any knowledge-retrieval node in the live workflow graph.

### Actual

The store is populated only once, on initial mount. Subsequent workflow-graph changes are not reflected.

### Suggested fix

Add `nodes` (and `updateDatasetsDetail` for completeness, even though it is already memoized with `[]` deps) to the effect's dependency array:

```tsx
useEffect(() => {
...
updateDatasetsDetail(allDatasetIds)
}, [nodes, updateDatasetsDetail])
```

Open web/app/components/workflow/datasets-detail-store/provider.tsx and inspect lines 34-46, especially the effect that reads nodes and updates the dataset-detail store. Exercise workflow graph changes such as loading another workflow or adding and removing knowledge-retrieval nodes. Done means the store reflects the dataset IDs referenced by the current graph rather than only the initial mount.

Written by the indexing model from the issue text.