Hello The-Lu,
Thanks for your reply.
Regarding “it doesn’t work on GitHub Enterprise (i.e., *.ghe.com)”: the extension appears to be restricted to "https://github.com/*", so rendering is not triggered on URLs such as "https://<mycompany>.ghe.com/*".
This may be due to my configuration or enterprise policy, but I can’t seem to change the list of enabled sites.
As for the rendering server, that was my mistake. I thought rendering happened on plantuml.com, but after rereading the documentation, I understand that it runs in a local JavaScript engine