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

src/app/stores/SnippetStore.ts

@@ -124,7 +124,11 @@ export const useSnippetStore = create<SnippetState>((set, get) => ({
 
   createSnippet: (options) => {
     get().commitDraft(); // flush the outgoing snippet's valid edits before switching away
-    const snippet = createSnippet(options);
+    const created = createSnippet(options);
+    // Mirror datasetRefs from the spec at creation, like publish does, so a snippet
+    // built with a named-data reference (Chart Builder, §06) is linked to its dataset
+    // immediately. Inline-data specs (the sample template) resolve to no refs.
+    const snippet = { ...created, datasetRefs: recomputeDatasetRefs(created.spec) };
     set((s) => ({
       snippets: [snippet, ...s.snippets],
       activeSnippetId: snippet.id,