fix(os): add fallback piece server start if systemd/OpenRC service fails
The systemd kiosk-piece-server.service may not start reliably on Fedora
live EROFS boot. kiosk-session.sh now starts the piece server directly
as a fallback if port 8080 isn't listening after the wait loop. Also
removes DefaultDependencies=no from the service for better systemd
ordering.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>