feat(web): single-language content authoring (LabelEditor + localized_text at default lang)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

web/src/authorities/authorities-page.tsx

@@ -31,7 +31,7 @@ export function AuthoritiesPage() {
   const onCreate = (event: FormEvent) => {
     event.preventDefault();
 
-    if (!labels.some((l) => l.lang === "en" && l.label)) {
+    if (!labels.some((l) => l.label)) {
       setError(true);
       return;
     }

web/src/authorities/authorities.test.tsx

@@ -25,7 +25,7 @@ test("lists authorities for the kind and creates one", async () => {
   );
   renderApp(tree(), { route: "/authorities/person" });
   expect(await screen.findByText("Ada Lovelace")).toBeInTheDocument();
-  await userEvent.type(screen.getByLabelText(/label \(en\)/i), "Carl von Linné");
+  await userEvent.type(screen.getByLabelText(/^label$/i), "Carl von Linné");
   await userEvent.click(screen.getByRole("button", { name: /create/i }));
   await waitFor(() => expect((body as { kind: string })?.kind).toBe("person"));
   expect((body as { labels: { label: string }[] }).labels[0].label).toBe("Carl von Linné");