Onboarding's "Local / custom endpoint" only wrote the OPENAI_BASE_URL env
var, which runtime resolution ignores — so a self-hosted endpoint was never
wired in and setup failed with "No usable credentials found for custom" even
though local servers need no key.
Route the local option through saveOnboardingLocalEndpoint: probe the
endpoint, auto-discover a model from /v1/models, persist provider=custom +
base_url + model via /api/model/set, then verify the runtime directly
(not via completeWithModelConfirm, which would re-assign the model without
base_url and wipe it). No onboarding form/UI changes — the existing single
URL field is enough.
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