Codex gate: _normalizeWebUIVersion suppressed empty/__WEBUI_VERSION__/'not detected'
but NOT 'unknown' — api/updates.py emits WEBUI_VERSION='unknown' on git-describe
failure (Docker/CI images), so a real client version would falsely fire the
stale-client banner against server='unknown'. Suppress 'unknown' (+ case-insensitive
for all sentinels). + regression test asserting unknown/UNKNOWN/Unknown don't fire.
