Remove scripts/setup_open_webui.sh and its 'one-command local bootstrap'
doc sections (EN + zh-Hans). The script pip-installed the third-party Open
WebUI frontend into ~/.local and managed a launchd/systemd user service —
a maintenance liability for downstream software we don't own, and the source
of the LAN first-admin signup footgun in #36121.
The Open WebUI *integration* via the OpenAI-compatible API server is
unaffected: the Docker/Docker-Compose setup, multi-user profile guide, and
troubleshooting in open-webui.md stay, and Open WebUI remains a listed
supported frontend. Only the install-and-service bootstrapper is gone.