fix: add error handling for admin login page and document admin credentials

- Add URL error parameter handling in login page
- Display appropriate error messages for different error types
- Document default admin credentials
- Provide security guidelines and password management instructions

src/app/admin/login/page.tsx

@@ -1,6 +1,6 @@
 'use client';
 
-import { useState, Suspense } from 'react';
+import { useState, useEffect, Suspense } from 'react';
 import { signIn } from 'next-auth/react';
 import { useRouter, useSearchParams } from 'next/navigation';
 import { Eye, EyeOff, Mail, Lock, AlertCircle, Loader2 } from 'lucide-react';
@@ -9,6 +9,7 @@ function LoginForm() {
   const router = useRouter();
   const searchParams = useSearchParams();
   const callbackUrl = searchParams.get('callbackUrl') || '/admin';
+  const urlError = searchParams.get('error');
   
   const [email, setEmail] = useState('');
   const [password, setPassword] = useState('');
@@ -16,6 +17,27 @@ function LoginForm() {
   const [error, setError] = useState('');
   const [loading, setLoading] = useState(false);
 
+  useEffect(() => {
+    if (urlError) {
+      switch (urlError) {
+        case 'Configuration':
+          setError('认证配置错误，请联系管理员');
+          break;
+        case 'AccessDenied':
+          setError('访问被拒绝，请检查权限');
+          break;
+        case 'Verification':
+          setError('验证失败，请重试');
+          break;
+        case 'CredentialsSignin':
+          setError('邮箱或密码错误');
+          break;
+        default:
+          setError('登录失败，请稍后重试');
+      }
+    }
+  }, [urlError]);
+
   const handleSubmit = async (e: React.FormEvent) => {
     e.preventDefault();
     setError('');