Extend friendly error copy to non-boundary (GlobalErrorHandler) transients

MARS-652's chooseErrorView/friendly copy only fires from React error boundaries (ErrorReportBlock). Unhandled promise rejections and other GlobalErrorHandler-captured transients (network error, Load failed, chunk-load-failed) never hit a boundary -- GlobalErrorHandler renders null, pure telemetry, no user-facing surface exists for these today. Whether to add one (toast?) is a product call, not assumed. Related to MARS-69 UX-friction epic. Deferred, not blocking the MARS-654-style alert-tier fold for items 1+5 of the 2026-08-21 batch triage.