Chart builder: discoverable entry points — Build Chart door, dataset picker, no-datasets state

src/app/modals/ModalCoordinator.ts

@@ -31,6 +31,12 @@ function snapshotOf(name: ModalName | null): string | null {
 
 /** Open `name`, optionally with a sub-target (dataset id, source key). */
 export function openModal(name: ModalName, arg?: string): void {
+  // TODO: setActiveModal fires the UrlStateSync subscribers before init() has
+  // loaded the target, so a navigable modal pushes one hash derived from the
+  // *previous* modal state (e.g. a stale `#datasets/dataset-N/build`) before the
+  // correct one — a spurious Back step. Swapping the two lines or suppressing
+  // sync until init completes would fix it; check init implementations don't
+  // assume the modal is already active.
   useAppStore.getState().setActiveModal(name);
   getModalConfig(name)?.init?.(arg);
   stateSnapshot = snapshotOf(name);

src/app/modals/UrlStateSync.ts

@@ -58,7 +58,7 @@ export function deriveViewState(): ViewState {
   if (modal && getModalConfig(modal)?.isUrlNavigable) {
     if (modal === 'chartBuilder') {
       const datasetId = useChartBuilderStore.getState().datasetId;
-      return datasetId !== null ? { kind: 'dataset-build', datasetId } : { kind: 'datasets' };
+      return datasetId !== null ? { kind: 'dataset-build', datasetId } : { kind: 'build' };
     }
     if (modal === 'datasets') {
       const ds = useDatasetStore.getState();
@@ -136,6 +136,13 @@ export function applyView(view: ViewState): void {
         app.setActiveModal('datasets');
         useDatasetStore.getState().startCreate();
         return;
+      case 'build':
+        // The no-target builder door. init(null) picks the freshest dataset, so
+        // with any datasets present the derived view immediately self-corrects to
+        // `dataset-build`; only an empty dataset library stays on `#build`.
+        useChartBuilderStore.getState().init(null);
+        app.setActiveModal('chartBuilder');
+        return;
     }
   } finally {
     applying = false;

src/app/modals/modal-registry.ts

@@ -71,8 +71,10 @@ const MODAL_REGISTRY: Partial<Record<ModalName, ModalConfig>> = {
     getState: () => ({ name: useExtractStore.getState().name }),
   },
 
-  // Opened from a selected dataset's "Build Chart" action; `arg` is its id. Loads
-  // the dataset and pre-populates a smart default config (§06). Applies on Create
+  // Opened from a dataset's "Build Chart" action (`arg` is its id) or from the
+  // library / onboarding doors with no arg — init(null) then picks the most
+  // recently modified dataset (§06 → Opening). Loads the dataset and
+  // pre-populates a smart default config (§06). Applies on Create
   // (a new snippet), so there is nothing transient to lose on close — no getState.
   // Backdrop dismissal is off: the config is real in-progress work, and a stray
   // click outside this large surface shouldn't throw it away (Escape/× still close).