ci: Add a GitHub workflow for UI building

Add a minimal workflow for building the UI, to automatically assess the
buildability of the UI codebase after Renovate Bot 's suggested
UI dependency updates.

Signed-off-by: Jyrki Keisala <[email protected]>

Author: Etsija

Diff for: .github/workflows/build-and-test.yml

@@ -34,6 +34,35 @@ jobs:
     - name: Build all classes
       run: ./gradlew --stacktrace classes
 
+    - name: Install Node
+      uses: actions/setup-node@v4
+      with:
+        node-version: 20
+
+    - uses: pnpm/action-setup@v3
+      name: Install pnpm
+      with:
+        version: 8
+        run_install: false
+
+    - name: Get pnpm store directory
+      shell: bash
+      run: |
+        echo "STORE_PATH=$(pnpm store path --silent)" >> $GITHUB_ENV
+
+    - uses: actions/cache@v4
+      name: Setup pnpm cache
+      with:
+        path: ${{ env.STORE_PATH }}
+        key: ${{ runner.os }}-pnpm-store-${{ hashFiles('**/pnpm-lock.yaml') }}
+        restore-keys: |
+          ${{ runner.os }}-pnpm-store-
+
+    - name: Install UI dependencies
+      run: |
+        cd ui
+        pnpm install
+
   test:
     needs: build
     runs-on: ubuntu-latest