refactor(layout, i18n, admin): 优化布局结构与多语言支持

调整 AdminShell 组件的子组件顺序，提升代码可读性。更新 admin-breadcrumb 组件，简化导航标签翻译逻辑，确保多语言支持的一致性。重构 admin-language-switcher 组件，优化语言切换的用户体验，增强界面交互性。更新多语言配置，新增登录界面的副标题，提升用户体验。

src/middleware.ts

@@ -0,0 +1,45 @@
+import { NextResponse, type NextRequest } from "next/server";
+
+import { ADMIN_TOKEN_STORAGE_KEY } from "@/lib/admin-token-constants";
+import { readAdminTokenFromCookieString } from "@/lib/admin-token-cookie";
+
+const ADMIN_LOGIN_PATH = "/admin/login";
+
+function isAdminLoginPath(pathname: string): boolean {
+  return pathname === ADMIN_LOGIN_PATH || pathname.startsWith(`${ADMIN_LOGIN_PATH}/`);
+}
+
+function readTokenFromRequest(request: NextRequest): string | null {
+  const fromCookie = request.cookies.get(ADMIN_TOKEN_STORAGE_KEY)?.value?.trim();
+  if (fromCookie) {
+    return fromCookie;
+  }
+
+  return readAdminTokenFromCookieString(request.headers.get("cookie"));
+}
+
+export function middleware(request: NextRequest) {
+  const { pathname } = request.nextUrl;
+
+  if (!pathname.startsWith("/admin")) {
+    return NextResponse.next();
+  }
+
+  if (isAdminLoginPath(pathname)) {
+    return NextResponse.next();
+  }
+
+  const token = readTokenFromRequest(request);
+  if (!token) {
+    const loginUrl = request.nextUrl.clone();
+    loginUrl.pathname = ADMIN_LOGIN_PATH;
+    loginUrl.search = "";
+    return NextResponse.redirect(loginUrl);
+  }
+
+  return NextResponse.next();
+}
+
+export const config = {
+  matcher: ["/admin", "/admin/:path*"],
+};