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

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

web/src/vocab/vocabularies.test.tsx

@@ -45,7 +45,7 @@ test("selecting a vocabulary shows its terms and adds one", async () => {
   );
   renderApp(tree(), { route: "/vocabularies/v-material" });
   expect(await screen.findByText("Bronze")).toBeInTheDocument();
-  await userEvent.type(screen.getByLabelText(/label \(en\)/i), "Stone");
+  await userEvent.type(screen.getByLabelText(/^label$/i), "Stone");
   await userEvent.click(screen.getByRole("button", { name: /add term/i }));
   await waitFor(() =>
     expect((termBody as { labels: { label: string }[] })?.labels[0].label).toBe("Stone"),

web/src/vocab/vocabulary-terms.tsx

@@ -34,7 +34,7 @@ export function VocabularyTerms() {
   const onAdd = (event: FormEvent) => {
     event.preventDefault();
 
-    if (!labels.some((l) => l.lang === "en" && l.label)) {
+    if (!labels.some((l) => l.label)) {
       setError(true);
       return;
     }