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>

pnpm-lock.yaml

@@ -117,6 +117,9 @@ importers:
       '@tanstack/react-query':
         specifier: ^5.96.1
         version: 5.96.1(react@19.2.4)
+      js-yaml:
+        specifier: ^4.1.1
+        version: 4.1.1
       react:
         specifier: ^19.2.4
         version: 19.2.4
@@ -130,6 +133,9 @@ importers:
       '@tailwindcss/vite':
         specifier: ^4.2.2
         version: 4.2.2(vite@8.0.3(@emnapi/core@1.9.1)(@emnapi/runtime@1.9.1)(@types/node@25.5.0)(esbuild@0.27.5)(jiti@2.6.1)(tsx@4.21.0))
+      '@types/js-yaml':
+        specifier: ^4.0.9
+        version: 4.0.9
       '@types/react':
         specifier: ^19.2.14
         version: 19.2.14
@@ -633,6 +639,9 @@ packages:
   '@types/http-errors@2.0.5':
     resolution: {integrity: sha512-r8Tayk8HJnX0FztbZN7oVqGccWgw98T/0neJphO91KkmOzug1KkofZURD4UaD5uH8AqcFLfdPErnBod0u71/qg==}
 
+  '@types/js-yaml@4.0.9':
+    resolution: {integrity: sha512-k4MGaQl5TGo/iipqb2UDG2UwjXziSWkh0uysQelTlJpX1qGlpUZYm8PnO4DxG1qBomtJUdYJ6qR6xdIah10JLg==}
+
   '@types/json-schema@7.0.15':
     resolution: {integrity: sha512-5+fP8P8MFNC+AyZCDxrB2pkZFPGzqQWUzpSeuuVLvm8VMcorNYavBqoFcxK8bQz4Qsbn4oUEEem4wDLfcysGHA==}
 
@@ -1830,6 +1839,8 @@ snapshots:
 
   '@types/http-errors@2.0.5': {}
 
+  '@types/js-yaml@4.0.9': {}
+
   '@types/json-schema@7.0.15': {}
 
   '@types/jsonwebtoken@9.0.10':