ci: Clean up UI builds from releases (#27706)

* ci : inline UI version resolution into ui-build.yml

* ci : build UI once and reuse the artifact in release jobs

Server jobs now extract the ui-build artifact into tools/ui/dist instead of npm-building the UI. Also removes the get-version job and the no-op -DHF_UI_VERSION flags.

Assisted-by: pi:Kimi-K3

* ui : disable the npm UI build by default (LLAMA_BUILD_UI=OFF)

The flag now only controls building the UI from source via npm. The UI
is still embedded by default from local tools/ui/dist or the prebuilt
download (LLAMA_USE_PREBUILT_UI=ON). CI jobs no longer npm-build the
UI; server-sanitize does not need node anymore.

Assisted-by: pi:Kimi-K3

* ci : rename the ui-build artifact to llama-ui.zip

Consistent with the other artifact names in the Actions summary.

Assisted-by: pi:Kimi-K3

* ci : clarify the windows artifact merge in release.yml

The windows-cuda/vulkan/sycl jobs build only the backend library;
llama-server (with the embedded UI) is injected into their zips from
the windows-cpu package during the release. State this in the job
comments and use accurate wording in the merge step.

Assisted-by: pi:Kimi-K3