If your WordPress installation is in a sub-directory of the domain, that’s only a concern to WordPress.
In the WordPress Settings page you have options to set the WordPress Address and the Site Address.
If you’re installation is in a sub-directory then the two values will be different, otherwise they will normally be the same.
Frontity is only interested in the REST API. WordPress will deliver the content from the domain defined in the Site Address setting. Frontity doesn’t know about the sub-directory, nor does it need to know about it.
Frontity will use the same URLs that appear in the address bar when you navigate the normal WordPress site. So, for example, you would use http://localhost:3000/blog to access the blog page.
The state.source.url setting should normally be the same as is defined in the Site Address setting.
You can test what the endpoint of the WordPress REST API is by entering http://yourdomain.com/wp-json/wp/v2 in the address bar of your browser, or in a tool like Postman. If you get content back then whatever you have used in place of http://yourdomain.com is what should be in state.source.url.