Distinguish between "provider rejects probe" (yellow warning) and
"genuinely broken" (red error) in health check results.
Backend: add AppError::HttpStatus variant to carry structured HTTP
status codes, populate http_status on error results, classify codes
into short labels (e.g. "Auth rejected (401)"), and truncate overly
long response bodies.
Frontend: route 401/403/400/429/5xx to toast.warning with localized
hints explaining the error may not indicate actual unusability; route
404/402/connection errors to toast.error. Add i18n keys for all three
locales (zh/en/ja).
Also deduplicate check_once by reusing build_stream_check_result.
Jason
689ca08409