feat: implement initial language resolution for SSR and local storage support

- Added a function to determine the initial language based on server-side rendering and local storage.
- Updated i18n initialization to use the resolved initial language instead of the default.

src/i18n/index.ts

@@ -53,6 +53,17 @@ const resources = {
   Record<(typeof namespaces)[number], Record<string, unknown>>
 >;
 
+function resolveInitialLanguage(): AppLanguage {
+  // SSR 始终使用默认语言，避免首屏渲染依赖浏览器状态
+  if (typeof window === "undefined") {
+    return DEFAULT_LANGUAGE;
+  }
+
+  const stored = window.localStorage.getItem("i18nextLng");
+  const preferred = normalizeLanguage(stored ?? window.navigator.language);
+  return preferred;
+}
+
 export function syncDocumentLanguage(lang: AppLanguage): void {
   if (typeof document === "undefined") return;
 
@@ -72,6 +83,8 @@ export function syncPreferredLanguage(): void {
 }
 
 if (!i18n.isInitialized) {
+  const initialLng = resolveInitialLanguage();
+
   void i18n.use(initReactI18next).init({
     resources,
     fallbackLng: DEFAULT_LANGUAGE,
@@ -80,8 +93,7 @@ if (!i18n.isInitialized) {
     ns: [...namespaces],
     /** 始终先用默认语言完成 SSR / 首次 hydration，避免首屏文本不一致 */
     load: "languageOnly",
-    lng: DEFAULT_LANGUAGE,
-    initImmediate: false,
+    lng: initialLng,
 
     interpolation: {
       escapeValue: false,