The OpenRouter PKCE flow only completed through the loopback callback
server, so remote/headless sessions (e.g. pi over SSH) could never
finish login: the browser runs on another machine and cannot reach
127.0.0.1 on the host running pi.
Race the callback server against a manual_code prompt, matching the
existing Anthropic and OpenAI Codex flows. Users can paste the final
redirect URL (or the bare authorization code) and the flow exchanges
it with the same PKCE verifier. A claimed callback keeps priority so
a pasted code cannot interrupt an in-flight exchange.
The callback server object follows the OAuthServerInfo shape from the
OpenAI Codex flow: waitForCredential() resolves null when cancelWait()
hands the login over to manual entry, and close() is pure cleanup that
never settles the wait.
Refs #7078
61da9e2f37