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/i18n/index.ts

@@ -0,0 +1,19 @@
+import type { i18n as I18nInstance } from 'i18next';
+
+import { getActiveI18nInstance } from '@/lib/translation';
+
+import { defaultLocale, supportedLocales, type SupportedLocale } from './constants';
+
+export { supportedLocales, defaultLocale } from './constants';
+export type { SupportedLocale } from './constants';
+export { localeCookieKey } from './constants';
+
+export const normalizeLocale = (locale?: string): SupportedLocale => {
+  if (!locale) return defaultLocale;
+  const normalized = locale.toLowerCase().split('-')[0] as SupportedLocale;
+  return supportedLocales.includes(normalized) ? normalized : defaultLocale;
+};
+
+export const getActiveI18n = (): I18nInstance | undefined => {
+  return getActiveI18nInstance();
+};