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/components/dashboard/StatsCard.vue

@@ -1,5 +1,5 @@
 <script setup lang="ts">
-import { useI18n } from 'vue-i18n';
+import { useTranslation } from 'i18next-vue';
 import { VNode } from 'vue';
 
 interface Trend {
@@ -19,7 +19,7 @@ withDefaults(defineProps<Props>(), {
   color: 'primary'
 });
 
-const { t } = useI18n();
+const { t } = useTranslation();
 
 // const gradients = {
 //   primary: 'from-primary/20 to-primary/5',