fix(02): resolve 3 typecheck errors and i18n langDir path

- useSetLocale → destructured setLocale from useI18n()
- addSeoAttributes → seo option for useLocaleHead()
- process.env → import.meta.env for Nuxt compatibility
- langDir: 'locales/' → 'app/locales/' (Nuxt 4 resolves from project root)

nuxt.config.ts

@@ -35,7 +35,7 @@ export default defineNuxtConfig({
       { code: 'fr', language: 'fr-FR', file: 'fr.json' },
       { code: 'en', language: 'en-US', file: 'en.json' },
     ],
-    langDir: 'locales/',
+    langDir: 'app/locales/',
     detectBrowserLanguage: {
       useCookie: true,
       cookieKey: 'i18n_redirected',
@@ -51,6 +51,6 @@ export default defineNuxtConfig({
   },
   gtag: {
     id: '',
-    enabled: process.env.NODE_ENV === 'production',
+    enabled: import.meta.env.NODE_ENV === 'production',
   }
 })