fix(build): swagger css import (#37801) (#37803)

Backport #37801 by @lunny

Snap build failure caused by missed swagger ui css file.

```
:: [plugin vite:css] /build/gitea/parts/gitea/build/web_src/css/swagger-standalone.css:undefined:NaN
:: Error: [postcss] ENOENT: no such file or directory, open '../../node_modules/swagger-ui-dist/swagger-ui.css'
```

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: silverwind <me@silverwind.io>

web_src/css/swagger-render.css

@@ -1,4 +1,4 @@
-@import "../../node_modules/swagger-ui-dist/swagger-ui.css";
+@import "swagger-ui-dist/swagger-ui.css";
 
 html,
 html body,