feat: 登录界面优化与API代理配置

- 移除登录页面的登录/注册切换，只保留登录功能
- 添加API代理配置支持远程访问
- 添加favicon.ico
- 后端CORS配置支持前端访问

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server.js

@@ -6,11 +6,14 @@ const cors = require('cors');
 const path = require('path');
 
 const app = express();
-const PORT = 3500;
+const PORT = 3501;
 const JWT_SECRET = process.env.JWT_SECRET || 'accountbook_secret_key_2024';
 
 // 中间件
-app.use(cors());
+app.use(cors({
+  origin: ['http://localhost:3500', 'http://127.0.0.1:3500'],
+  credentials: true
+}));
 app.use(express.json());
 app.use(express.static(path.join(__dirname, 'public')));
 app.use(express.static(path.join(__dirname, 'frontend/.next')));