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/ui-build.yml

@@ -3,8 +3,8 @@ name: UI Build
 on:
   workflow_call:
     inputs:
-      hf_ui_version:
-        description: 'Version string for version.json (e.g. 12345)'
+      ui_version:
+        description: 'Version string embedded in build.json (e.g. b1234); defaults to b<commit-count>'
         required: false
         type: string
 
@@ -17,6 +17,17 @@ jobs:
     steps:
       - name: Checkout code
         uses: actions/checkout@v6
+        with:
+          fetch-depth: 0
+
+      - name: Resolve UI version
+        id: version
+        run: |
+          version="${{ inputs.ui_version }}"
+          if [ -z "$version" ]; then
+            version="b$(git rev-list --count HEAD)"
+          fi
+          echo "ui_version=${version}" >> $GITHUB_OUTPUT
 
       - name: Setup Node.js
         uses: actions/setup-node@v6
@@ -31,8 +42,7 @@ jobs:
 
       - name: Build application
         env:
-          HF_UI_VERSION: ${{ inputs.hf_ui_version || '' }}
-          LLAMA_BUILD_NUMBER: ${{ inputs.hf_ui_version || 'b0000' }}
+          LLAMA_BUILD_NUMBER: ${{ steps.version.outputs.ui_version }}
         run: npm run build
         working-directory: tools/ui
 
@@ -43,6 +53,6 @@ jobs:
       - name: Upload built UI
         uses: actions/upload-artifact@v6
         with:
-          name: ui-build
+          name: llama-ui.zip
           path: tools/ui/dist/
           retention-days: 1