web-next: página 404 temática (not-found)

Añade una 404 con el markup del tema (main-page > middle-content >
body-content > title-content + body-box-content justified), dentro del
layout de [locale] (cabecera/vídeo/pie). Como el layout raíz es un segmento
dinámico ([locale]), se usa el patrón de next-intl: `[locale]/not-found.tsx`
+ catch-all `[locale]/[...rest]/page.tsx` que dispara notFound(). Bilingüe
(claves NotFound.title/message en es/en). Devuelve HTTP 404; las rutas
conocidas siguen 200.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

web-next/messages/en.json

@@ -510,5 +510,9 @@
     "backToStore": "Back to the store",
     "orderNotFound": "Order not found or already processed.",
     "genericError": "An error occurred. Please try again."
+  },
+  "NotFound": {
+    "title": "Page not found",
+    "message": "It looks like the page you are looking for doesn't exist or isn't available."
   }
 }