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)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/app.vue

@@ -1,6 +1,6 @@
 <script setup lang="ts">
 const { locale } = useI18n()
-const head = useLocaleHead({ addSeoAttributes: true })
+const head = useLocaleHead({ seo: true })
 
 useHead({
   htmlAttrs: { lang: locale },