the open page lists its bands in the nav

A third level, under the selected page only — every page has bands, and listing
all of them would make the nav longer than the site.

Announced by the builder rather than fetched again: it already reads them to know
what it is looking at in the document, and it mounts exactly when a page is open.
Cleared on unmount, or the nav keeps listing the last page's bands.

Clicking one writes a third URL segment and the preview scrolls to it, keyed on
the render count too so the position survives a save replacing the frame.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

web/usePages.ts

@@ -60,3 +60,32 @@ export const pageFromParam = (pages: Page[] | null, value: string | null): Page
   if (!pages || !value) return null;
   return pages.find((page) => pageParam(page) === value) ?? null;
 };
+
+/**
+ * The selected page's sections, shared between the builder and the nav.
+ *
+ * ── Announced, not fetched twice ──
+ *
+ * The page builder already reads them — it needs their names and columns to know what it is looking at
+ * in the rendered document — and the nav draws them as rows under the open page. Two fetchers would
+ * agree until somebody renamed a band.
+ *
+ * The builder mounts whenever a page is selected, and the nav only draws these under the page that is
+ * selected, so there is no state where the nav wants them and nothing has spoken.
+ *
+ * Null means nobody has said yet: the nav then draws no rows rather than an empty list, which would
+ * read as "this page has no sections".
+ */
+export type NamedSection = { id: number; name: string };
+
+const useSectionsList = defineChannel<NamedSection[] | null>('fofos-admin:sections', null);
+
+export const useSections = () => {
+  const [sections, setSections] = useSectionsList();
+
+  const setRef = useRef(setSections);
+  setRef.current = setSections;
+  const announceSections = useCallback((rows: NamedSection[] | null) => setRef.current(rows), []);
+
+  return { sections, announceSections };
+};