Additional OAuth login via device flow following current pattern (working "crush login xai" ready)

[bellhyve]

Howdy, Crush team!

I was looking at PR 2973: xAI OAuth support and realized this is simple to implement now that we can follow the existing OAuth pattern.

I used Hyper to make a commit, then reviewed with Grok 4.5 and my old eyeballs. It passed the lint and build tests on my personal branch.

It works well, and I'll continue production testing. I'm also happy to add additional OAuth support for other services based on your pattern.

Best,
Daniel