The login screen already shows a yellow banner when /api/* is in
proxy-down (502/504) or auth-down (503) state. Extend the same signal
to every authenticated scene so a regional store manager seeing a
broken Mission Control gets a one-line explanation, not silent failure.
- New src/lib/useBackendHealth.ts polls /api/v1/auth/me every 30s
(cheap, returns 401 when healthy + unauth'd so it doubles as auth
probe). Status maps: 200/401/403 -> up, 502/504 -> proxy-down,
503 -> auth-down.
- App.tsx renders .global-banner above the topbar whenever the proxy
or auth service is down. Hidden on /login (Login.tsx already has
its own banner) and during SSO callback.
- Dark-theme banner colours added.
31/31 vitest pass. tsc + vite build green.
