refactor: OpenAPI URL 抓取改为前端执行 + 服务端 CORS 代理

- 前端直接 fetch URL 支持 localhost/内网地址
- CORS 失败自动回退到服务端代理 /api/fetch-spec
- 添加 js-yaml 支持 YAML 格式解析
- 服务端移除 specUrl 参数，只接收已解析的 spec 对象

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

packages/web/package.json

@@ -10,6 +10,7 @@
   },
   "devDependencies": {
     "@tailwindcss/vite": "^4.2.2",
+    "@types/js-yaml": "^4.0.9",
     "@types/react": "^19.2.14",
     "@types/react-dom": "^19.2.3",
     "@vitejs/plugin-react": "^6.0.1",
@@ -19,6 +20,7 @@
   },
   "dependencies": {
     "@tanstack/react-query": "^5.96.1",
+    "js-yaml": "^4.1.1",
     "react": "^19.2.4",
     "react-dom": "^19.2.4",
     "react-router-dom": "^7.13.2"