When the /devices fetch hasn't completed or fails, parseInt on an empty
select returns NaN which JSON-serializes to null. The backend then calls
int(None) and raises TypeError. Fix both layers: frontend falls back to
0 on NaN, backend uses `or` defaults so null values don't bypass the
fallback.
Also adds a short TTL cache to detect_all_devices() so multiple
concurrent callers on the same page load don't each spawn blocking
subprocess probes.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
2f5f429e83