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>

frontend/pages/api/[...path].js

@@ -0,0 +1,17 @@
+export default async function handler(req, res) {
+  const { 0: path } = req.query;
+
+  const backendUrl = `http://localhost:3501/${path}`;
+
+  const response = await fetch(backendUrl, {
+    method: req.method,
+    headers: {
+      'Content-Type': 'application/json',
+      ...(req.headers.authorization ? { Authorization: req.headers.authorization } : {}),
+    },
+    body: req.method !== 'GET' && req.method !== 'HEAD' ? JSON.stringify(req.body) : undefined,
+  });
+
+  const data = await response.json();
+  res.status(response.status).json(data);
+}