feat: 添加国际化支持，优化语言切换器和登录界面，增强用户体验

src/components/i18n-hydration-provider.tsx

@@ -0,0 +1,25 @@
+"use client";
+
+import { createContext, useContext, useEffect, useState, type ReactNode } from "react";
+
+import { syncPreferredLanguage } from "@/i18n";
+
+const I18nHydrationContext = createContext(false);
+
+/** hydration 完成前保持 DEFAULT_LANGUAGE，与 SSR 一致；完成后再同步用户偏好语言。 */
+export function I18nHydrationProvider({ children }: { children: ReactNode }) {
+  const [hydrated, setHydrated] = useState(false);
+
+  useEffect(() => {
+    syncPreferredLanguage();
+    setHydrated(true);
+  }, []);
+
+  return (
+    <I18nHydrationContext.Provider value={hydrated}>{children}</I18nHydrationContext.Provider>
+  );
+}
+
+export function useI18nHydrated(): boolean {
+  return useContext(I18nHydrationContext);
+}