feat(i18n): initialize i18n infrastructure

i18next.config.ts

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+export default {
+  // supported languages
+  locales: ["en", "zh", "de", "fr"],
+
+  // extraction config (for i18next-cli)
+  extract: {
+    input: "src/**/*.{js,jsx,ts,tsx}",
+    output: "public/locales/{{language}}/{{ns}}.json",
+    namespaceSeparator: false,
+  },
+
+  // default namespace
+  defaultNamespace: "translation",
+
+  // output path pattern
+  resource: {
+    jsonIndent: 2,
+    lineEnding: "\n",
+  },
+
+  // sort keys in output
+  sort: true,
+
+  // fail on missing keys during dev
+  // quiet: false,
+
+  // metadata for translation files
+  metadata: {
+    description: "Generated by i18next-cli",
+    generatedAt: new Date().toISOString(),
+  },
+};

src/lib/i18n.ts

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+import i18n from 'i18next';
+import {initReactI18next} from 'react-i18next';
+import HttpBackend from 'i18next-http-backend';
+import LanguageDetector from 'i18next-browser-languagedetector';
+
+i18n
+  .use(HttpBackend)
+  .use(LanguageDetector)
+  .use(initReactI18next)
+  .init({
+    fallbackLng: 'en',
+    supportedLngs: ['en', 'zh', 'de', 'fr'],
+    defaultNS: 'translation',
+    ns: ['translation'],
+    backend: {
+      loadPath: '/locales/{{lng}}/{{ns}}.json',
+    },
+    detection: {
+      order: ['localStorage', 'navigator'],
+      caches: ['localStorage'],
+      lookupLocalStorage: 'i18nextLng',
+    },
+    interpolation: {
+      escapeValue: false,
+    },
+  });
+
+export default i18n;

src/lib/language-provider.tsx

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+import {useQuery} from '@tanstack/react-query';
+import {type ReactNode, useEffect} from 'react';
+import {getPreferences} from '@/client';
+import {useUser} from '@/contexts';
+import i18n from '@/lib/i18n';
+
+/**
+ * Syncs the user's server-side language preference to i18n after login.
+ * This ensures the API-stored preference takes precedence over browser detection.
+ */
+export function LanguageProvider({children}: {children: ReactNode}) {
+  const {isAuthenticated} = useUser();
+
+  const {data: preferences} = useQuery({
+    queryKey: ['userPreferences'],
+    queryFn: async () => {
+      const resp = await getPreferences();
+      return resp.data?.data ?? null;
+    },
+    enabled: isAuthenticated,
+    staleTime: 5 * 60 * 1000,
+    retry: false,
+  });
+
+  useEffect(() => {
+    if (preferences?.language && preferences.language !== i18n.language) {
+      i18n.changeLanguage(preferences.language);
+    }
+  }, [preferences]);
+
+  return children;
+}