refactor(seo): migrate metadata handling to Nuxt useHead

Remove direct document.head mutations to support SSR compatibility
Implement observer pattern to sync SEO state with Nuxt universal plugin
Update analytics script to use declarative injection in Nuxt config

frontend/nuxt.config.ts

@@ -62,8 +62,9 @@ export default defineNuxtConfig({
       ],
       script: [
         {
-          innerHTML:
-            '(function(){const s=document.createElement("script");s.async=true;s.src="https://umami.tootaio.com/script.js";s.setAttribute("data-website-id","6c00a2e5-dc72-41f3-9d5d-aac93aaaf1cb");document.head.appendChild(s);})();'
+          async: true,
+          src: 'https://umami.tootaio.com/script.js',
+          'data-website-id': '6c00a2e5-dc72-41f3-9d5d-aac93aaaf1cb'
         }
       ]
     }