Add provider login to RPC mode

### What do you want to change? Add an RPC login operation backed by ModelRuntime.login(). Expose available OAuth/API-key methods and forward AuthPrompt and AuthEvent data to the client. ### Why? /login only works in interactive mode. RPC clients must currently restart with credentials, edit auth.json, or launch a separate Pi TUI to add a provider. ### How? (optional) _No response_