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/components/layout/AppHeader.vue

@@ -1,7 +1,6 @@
 <script setup lang="ts">
-const { t, locale } = useI18n()
+const { t, locale, setLocale } = useI18n()
 const localePath = useLocalePath()
-const setLocale = useSetLocale()
 const colorMode = useColorMode()
 const route = useRoute()
 const drawerOpen = ref(false)