feat: generate /fr/sitemap.xml via custom Nitro route

@nuxtjs/sitemap is incompatible with app.baseURL: '/fr/' here — it broke
nginx routing (403) on this deployment where /fr/ is served via an alias
to a separate static build. A hand-rolled Nitro route sidesteps the
module entirely and gets prerendered to a plain sitemap.xml file at the
root of the generated output, matching what the nginx alias expects.

nuxt.config.ts

@@ -44,5 +44,11 @@ export default defineNuxtConfig({
     }
   },
 
+  nitro: {
+    prerender: {
+      routes: ['/sitemap.xml']
+    }
+  },
+
   compatibilityDate: '2024-10-24'
 })