OpenRouter connects with a browser sign-in

OpenRouter can now be connected from the browser. In /setup, pick OpenRouter and then Sign in with browser: Yolop opens openrouter.ai/auth, waits on a loopback callback, exchanges the code for an API key and stores it the same way as a pasted key. ACP clients see the method as openrouter_browser.

A pasted key and OPENROUTER_API_KEY still work, and the environment variable wins when both are set.

See Providers and models and #567.