Reapply "Add missing admin action files and navigation links"

This reverts commit 4d515bc400.

src/app/admin/translations/page.tsx

@@ -0,0 +1,7 @@
+import { redirect } from 'next/navigation'
+
+// Default landing for /admin/translations → first tab.
+// The shared TranslationsTabs (in layout.tsx) gives access to every section.
+export default function TranslationsHubPage() {
+  redirect('/admin/translations/cms')
+}