fix: use constant for cookie value and remove unused import

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

middleware.ts

@@ -1,7 +1,7 @@
 // middleware.ts
 import { NextResponse } from 'next/server';
 import type { NextRequest } from 'next/server';
-import { COOKIE_NAME, COOKIE_MAX_AGE } from './lib/constants';
+import { COOKIE_NAME, AUTH_COOKIE_VALUE } from './lib/constants';
 
 export function middleware(request: NextRequest) {
   const { pathname } = request.nextUrl;
@@ -14,7 +14,7 @@ export function middleware(request: NextRequest) {
   // 检查是否有认证 cookie
   const authCookie = request.cookies.get(COOKIE_NAME);
 
-  if (!authCookie || authCookie.value !== 'ok') {
+  if (!authCookie || authCookie.value !== AUTH_COOKIE_VALUE) {
     // 未认证，重定向到登录页
     const loginUrl = new URL('/login', request.url);
     return NextResponse.redirect(loginUrl);