ui : make LLAMA_BUILD_UI opt-in (default OFF)
No UI is provisioned or embedded unless -DLLAMA_BUILD_UI=ON is passed.
When ON, the provisioning chain is: local tools/ui/dist -> npm build ->
prebuilt download.
Enable it in the release CI jobs that embed the UI and in the Docker
images. server-sanitize no longer needs node.
Assisted-by: pi:Kimi-K3
