Redirect /\[lang\] to /\[lang\]/onlineboard

/ru rendered a blank page because no index route existed under
src/routes/\[lang\]/. Angular's app-routing.module.ts redirects any
empty top-level path to /onlineboard; mirror that in React by adding
a tiny <Navigate replace> index route.

src/routes/[lang]/page.tsx

@@ -0,0 +1,15 @@
+/**
+ * Root locale route — redirects `/ru` → `/ru/onlineboard`.
+ *
+ * Matches Angular's app-routing.module.ts where any top-level request
+ * resolves to the online-board start page. Using <Navigate> keeps the
+ * redirect working under both SSR and CSR since react-router's
+ * Navigate component renders during hydration too.
+ */
+
+import { Navigate, useParams } from "@modern-js/runtime/router";
+
+export default function LangRoot(): JSX.Element {
+  const { lang } = useParams<{ lang: string }>();
+  return <Navigate to={`/${lang ?? "ru"}/onlineboard`} replace />;
+}