replace vue-i18n with i18next-vue

Complete the i18n migration by switching runtime setup and remaining components to i18next-vue, and add shared locale constants/helpers for SSR and client language handling.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/routes/NotFound.vue

@@ -9,7 +9,7 @@
 
 <script setup lang="ts">
 import { VueHead } from '@/components/VueHead';
-import { useI18n } from 'vue-i18n';
+import { useTranslation } from 'i18next-vue';
 
-const { t } = useI18n();
+const { t } = useTranslation();
 </script>