xAI Grok was only reachable via the "I have an API key" form. xAI's
OAuth (SuperGrok / Premium+) flow already exists in the backend
(`hermes auth add xai-oauth`) but was never surfaced in the desktop
onboarding launcher.
Add a loopback PKCE flow: the local backend binds the 127.0.0.1
callback listener, the client opens the browser, and the redirect lands
back automatically — no code to copy/paste. Reuses the existing xAI
OAuth helpers (discovery, callback server, token exchange, persist)
rather than duplicating them.
- web_server: catalog entry (flow: loopback) + status dispatch +
_start_xai_loopback_flow + background worker + route branch
- desktop: 'loopback' flow type, awaiting_browser status, xAI Grok card
(PROVIDER_DISPLAY / FLOW_SUBTITLES / FlowPanel waiting render)
- tests: catalog listing, start authorize-url, worker persist, state
mismatch rejection
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