feat(admin): enhance login form with language switcher and update script handling

- Added AdminLanguageSwitcher to the login form for improved language selection.
- Replaced inline script with next/script component for better performance and safety in layout.tsx.
- Updated language initialization logic to ensure consistent SSR and client rendering.

src/i18n/index.ts

@@ -6,7 +6,6 @@ import { initReactI18next } from "react-i18next";
 import {
   adminHtmlLang,
   applyAdminUiLocale,
-  getStoredAdminUiLocale,
   hydrateAdminUiLocale,
   type AdminApiLocale,
 } from "@/lib/admin-locale";
@@ -124,16 +123,9 @@ export function normalizeAdminLanguage(lang: string | undefined): AdminLanguage
 }
 
 function getInitialAdminLanguage(): AdminLanguage {
-  if (typeof window === "undefined") {
-    return ADMIN_DEFAULT_LANGUAGE;
-  }
-
-  const stored = getStoredAdminUiLocale();
-  if (stored) {
-    return stored;
-  }
-
-  return normalizeAdminLanguage(document.documentElement.lang);
+  // Keep SSR and first client render aligned to avoid hydration mismatch.
+  // Stored locale is applied after mount in `Providers`.
+  return ADMIN_DEFAULT_LANGUAGE;
 }
 
 function syncAdminLanguage(lang: AdminLanguage): void {