[Studio][Bug] Render unknown consumer lag as unavailable

## Problem The backend uses -1 for unknown lag, but the UI renders it as a green number and an online Group is judged as running. Locations: `web/src/pages/studio/GroupManagement.tsx`, `web/src/pages/instance/consumer.tsx`. ## Expected behavior Share one isLagAvailable/formatting helper; render unknown values as "unavailable", exclude them from backlog-threshold and health-color decisions, and sort them after valid data. ## Verification Cover -1, 0, normal backlog and over-threshold backlog. Note: #1772 fixed the backend aggregation; the UI semantics here were not handled.