Pre-checks /api/v1/auth/me on mount. The status code IS the signal:
- 200/401/403 → backend up, login can succeed
- 502/504 → proxy down (canvas nginx → demo unreachable)
- 503 → auth service is down upstream
Surfaces a yellow banner above the form when proxy or auth is down so
a regional store manager doesn't type their password into a flow that's
guaranteed to fail. Matches the friendlyAuthError() copy from PR #3.
31/31 vitest tests pass.