perf: lazy-load locale files per language instead of bundling all

Split 574KB monolithic locales chunk into per-language lazy chunks:
- ru: 211KB (loaded on startup as fallback)
- fa: 143KB, en: 121KB, zh: 101KB (loaded on demand)

Russian users save 63% bandwidth. Other languages loaded only when
user switches via LanguageSwitcher.

src/i18n.ts

@@ -2,25 +2,36 @@ import i18n from 'i18next';
 import { initReactI18next } from 'react-i18next';
 import LanguageDetector from 'i18next-browser-languagedetector';
 
-import ru from './locales/ru.json';
-import en from './locales/en.json';
-import zh from './locales/zh.json';
-import fa from './locales/fa.json';
-
-const resources = {
-  ru: { translation: ru },
-  en: { translation: en },
-  zh: { translation: zh },
-  fa: { translation: fa },
+const localeLoaders: Record<string, () => Promise<{ default: Record<string, string> }>> = {
+  ru: () => import('./locales/ru.json'),
+  en: () => import('./locales/en.json'),
+  zh: () => import('./locales/zh.json'),
+  fa: () => import('./locales/fa.json'),
 };
 
+const SUPPORTED_LANGS = Object.keys(localeLoaders);
+const FALLBACK_LNG = 'ru';
+
+const loadedLanguages = new Set<string>();
+
+async function loadLanguage(lng: string): Promise<void> {
+  if (loadedLanguages.has(lng)) return;
+
+  const loader = localeLoaders[lng];
+  if (!loader) return;
+
+  const mod = await loader();
+  i18n.addResourceBundle(lng, 'translation', mod.default, true, true);
+  loadedLanguages.add(lng);
+}
+
 i18n
   .use(LanguageDetector)
   .use(initReactI18next)
   .init({
-    resources,
-    fallbackLng: 'ru',
-    supportedLngs: ['ru', 'en', 'zh', 'fa'],
+    fallbackLng: FALLBACK_LNG,
+    supportedLngs: SUPPORTED_LANGS,
+    partialBundledLanguages: true,
 
     detection: {
       order: ['localStorage', 'navigator'],
@@ -37,4 +48,15 @@ i18n
     },
   });
 
+// Load detected language + fallback on startup
+const detectedLng = i18n.language?.split('-')[0] || FALLBACK_LNG;
+const langsToLoad = [FALLBACK_LNG, ...(detectedLng !== FALLBACK_LNG ? [detectedLng] : [])];
+Promise.all(langsToLoad.map(loadLanguage));
+
+// Lazy-load on language change
+i18n.on('languageChanged', (lng: string) => {
+  const code = lng.split('-')[0];
+  loadLanguage(code);
+});
+
 export default i18n;