SAML post to correct URL and make example run locally

[adricasti]

Hi Ross, very useful project, I made three changes to make the example work:

• The IdP should post the authentication form to URLSSO, not URLLogin, otherwise the flow stops there
• I fixed the URLs for the example service so it runs in localhost to start with and without conflicting ports for SP and IdP
• The ServeMux in Go 1.24+ can't identify correct routes and we also need to be able to POST to /saml

[crewjam]

This is looking good. I'm not sure if it is quite ready to merge yet, but I'm down when it is. Let me know if you want more detailed feedback :)