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/notification/components/NotificationList.vue

@@ -1,5 +1,5 @@
 <script setup lang="ts">
-import { useI18n } from 'vue-i18n';
+import { useTranslation } from 'i18next-vue';
 import NotificationItem from './NotificationItem.vue';
 
 interface Notification {
@@ -24,7 +24,7 @@ const emit = defineEmits<{
     delete: [id: string];
 }>();
 
-const { t } = useI18n();
+const { t } = useTranslation();
 </script>
 
 <template>