refactor: remove LocaleSwitcher component and update locale handling

- Removed LocaleSwitcher from NavUser, SiteHeader, and WorkbenchHeader components.
- Updated tests to reflect the removal of LocaleSwitcher.
- Changed default locale from "en-US" to "zh-CN" in i18n configuration.
- Refactored locale resolution logic to read configured locale without relying on browser language detection.
- Updated AgentDesk SDK to support language configuration from public API.
- Cleaned up unused auth options fetching in the auth API.

web/app/dashboard/login/page.tsx

@@ -1,4 +1,3 @@
-import { LocaleSwitcher } from "@/components/locale-switcher"
 import { LoginForm } from "@/components/login-form"
 import { Suspense } from "react"
 
@@ -6,9 +5,6 @@ export default function LoginPage() {
   return (
     <div className="flex min-h-svh flex-col items-center justify-center bg-muted p-6 md:p-10">
       <div className="w-full max-w-sm md:max-w-4xl">
-        <div className="mb-4 flex justify-end">
-          <LocaleSwitcher />
-        </div>
         <Suspense fallback={<div className="min-h-96" />}>
           <LoginForm />
         </Suspense>