feat: user frontend i18n (zh-CN/en)

- vue-i18n@9 with locale files for zh-CN and en
- Language switcher in topbar
- Navigation, breadcrumb, credits page translated
- Discovery page i18n keys prepared
- Language persisted in localStorage
- Build verified

user-frontend/src/i18n.js

@@ -0,0 +1,22 @@
+import { createI18n } from 'vue-i18n'
+import zhCN from './locales/zh-CN.json'
+import en from './locales/en.json'
+
+const savedLang = localStorage.getItem('lang') || 'zh-CN'
+
+const i18n = createI18n({
+  legacy: false,
+  locale: savedLang,
+  fallbackLocale: 'zh-CN',
+  messages: {
+    'zh-CN': zhCN,
+    'en': en,
+  },
+})
+
+export function switchLang(lang) {
+  i18n.global.locale.value = lang
+  localStorage.setItem('lang', lang)
+}
+
+export default i18n