feat(frontend): enable Nuxt SSR and migrate to Nitro server

Set `ssr: true` in Nuxt config and switch build command to `nuxt build`.
Update Dockerfile to run `.output/server/index.mjs` and remove static server.
Defer SEO initialization to prevent premature evaluation during SSR.

frontend/nuxt.config.ts

@@ -5,7 +5,7 @@ function normalizeSiteUrl(value: string | undefined): string {
 }
 
 export default defineNuxtConfig({
-  ssr: false,
+  ssr: true,
   devtools: { enabled: false },
   css: ['~/src/styles/main.css'],
   compatibilityDate: '2026-05-06',