feat(i18n): 增强管理端多语言支持，优化语言初始化与恢复逻辑

src/lib/admin-locale.ts

@@ -69,7 +69,8 @@ export function adminHtmlLang(loc: AdminApiLocale): string {
   return "en";
 }
 
-function readStoredUiLocale(): AdminApiLocale | null {
+/** 读取用户上次选择的界面语言（无则 null） */
+export function getStoredAdminUiLocale(): AdminApiLocale | null {
   if (typeof window === "undefined") {
     return null;
   }
@@ -104,7 +105,7 @@ export function hydrateAdminUiLocale(): AdminApiLocale | null {
   if (typeof window === "undefined") {
     return null;
   }
-  const stored = readStoredUiLocale();
+  const stored = getStoredAdminUiLocale();
   if (stored) {
     setAdminRequestLocale(stored);
     document.documentElement.lang = adminHtmlLang(stored);