Fix BASE_PATH configuration breaking login in development mode

[Copilot]

When BASE_PATH is configured (e.g., BASE_PATH=/mcphub/), login fails in development because the Vite proxy doesn't match BASE_PATH-prefixed requests, and trailing slashes create invalid double-slash routes.

Changes

• Backend (src/config/index.ts): Normalize BASE_PATH by stripping trailing slashes
• Frontend (frontend/vite.config.ts): Generate Vite proxy rules dynamically from BASE_PATH environment variable
• Tests (tests/integration/base-path-routes.test.ts): Verify BASE_PATH normalization and routing behavior
• Docs (docs/BASE_PATH_CONFIGURATION.md): Usage guide with Docker/nginx examples

Implementation

// Backend: Normalize BASE_PATH
const normalizeBasePath = (path: string): string => 
  path ? path.replace(/\/+$/, '') : '';

// Frontend: Dynamic proxy from BASE_PATH
const env = loadEnv(mode, path.resolve(__dirname, '..'), '');
const basePath = env.BASE_PATH?.replace(/\/+$/, '') || '';
const proxyConfig = ['/api', '/config', '/public-config', '/health', '/oauth']
  .reduce((acc, path) => ({ ...acc, [basePath + path]: { target: 'http://localhost:3000' } }), {});

Fixes deployment under subdirectories and reverse proxies with path prefixes. Backward compatible with existing deployments.

Original prompt

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This section details on the original issue you should resolve

<issue_title>本地开发模式配置BASE_PATH后无法登陆了</issue_title>
<issue_description>Bug Description / 问题描述
What happened? / 发生了什么？

Steps to Reproduce / 复现步骤

1. 修改env文件，将BASE_PATH设置为"/mcphub/"
2. 重启应用
3. 发现登陆异常

Expected Behavior / 预期行为
希望能正常登陆

Environment / 运行环境

• Running on / 运行方式: [docker/npx/local / docker/npx/本地]
• Version / 版本: 0.10.3

Screenshots / 截图
If relevant, add screenshots / 如果有帮助的话，请添加截图

Additional Info / 补充信息
F12调试发现配置了BASE_PATH后，/api/，/login/以及一些其他鉴权相关的url不会自动加上basepath前缀
</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes 本地开发模式配置BASE_PATH后无法登陆了 #425

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