refactor(i18n): isolate Vue I18n instances per request for SSR

Replace global I18n singleton with a factory function
Inject request-specific I18n instances into Nuxt app and SEO metadata
Prevent cross-request locale state pollution during server-side rendering

frontend/plugins/01-i18n.ts

@@ -1,5 +1,15 @@
-import { i18n } from '../src/i18n';
+import { createPokopiaI18n, setActiveI18n } from '../src/i18n';
 
 export default defineNuxtPlugin((nuxtApp) => {
+  const i18n = createPokopiaI18n();
+  if (import.meta.client) {
+    setActiveI18n(i18n);
+  }
+
   nuxtApp.vueApp.use(i18n);
+  return {
+    provide: {
+      pokopiaI18n: i18n
+    }
+  };
 });