fix: 移除 usage-doc 页面重复的语言切换器

- 移除 page.tsx 中固定定位的语言切换器组件
- layout.tsx 中的 DashboardHeader 已包含语言切换器
- 移除未使用的 LanguageSwitcher 导入

src/app/[locale]/usage-doc/page.tsx

@@ -8,7 +8,6 @@ import { Button } from "@/components/ui/button";
 import { Tabs, TabsList, TabsTrigger, TabsContent } from "@/components/ui/tabs";
 import { TocNav, type TocItem } from "./_components/toc-nav";
 import { QuickLinks } from "./_components/quick-links";
-import { LanguageSwitcher } from "@/components/ui/language-switcher";
 
 const headingClasses = {
   h2: "scroll-m-20 text-2xl font-semibold leading-snug text-foreground",
@@ -1542,11 +1541,6 @@ export default function UsageDocPage() {
         {t("skipLinks.tableOfContents")}
       </a>
 
-      {/* Language Switcher - Fixed position */}
-      <div className="fixed top-4 right-4 z-50 lg:top-6 lg:right-8">
-        <LanguageSwitcher size="sm" />
-      </div>
-
       <div className="relative flex gap-6 lg:gap-8">
         {/* 左侧主文档 */}
         <div className="flex-1 min-w-0">