chatPublic() calls the provider's /chat/completions directly from the
renderer. Verified live: a saved profile pointing at
platform.deepseek.com (DeepSeek's console) instead of api.deepseek.com
(their actual API) fails the CORS preflight outright - 403, no
Access-Control-* headers - which surfaces to fetch() as an
undifferentiated "Failed to fetch" with no status to inspect. Confirmed
the real API and OpenRouter both support being called directly from a
browser fine, so the architecture is sound; only the error message was
useless. Now names the URL and the likely cause instead.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
