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

.github/workflows/docker.yml

@@ -64,7 +64,7 @@ jobs:
     needs: create_tag
     uses: ./.github/workflows/ui-build.yml
     with:
-      hf_ui_version: ${{ needs.create_tag.outputs.source_tag }}
+      ui_version: ${{ needs.create_tag.outputs.source_tag }}
 
   prepare_matrices:
     name: Prepare Docker matrices
@@ -162,7 +162,7 @@ jobs:
         if: ${{ matrix.config.prebuilt_ui == true }}
         uses: actions/download-artifact@3e5f45b2cfb9172054b4087a40e8e0b5a5461e7c # v8
         with:
-          name: ui-build
+          name: llama-ui.zip
           path: tools/ui/dist
 
       - name: Set up QEMU