Add CSS build step to deployment workflow

The deployment workflow also needs to build CSS before Jekyll generates
the site, since compiled.css is no longer in the repository.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: hsbt

.github/workflows/jekyll.yml

@@ -27,6 +27,15 @@ jobs:
         with:
           ruby-version: '3.2'
           bundler-cache: true
+      - name: Setup Node.js
+        uses: actions/setup-node@v4
+        with:
+          node-version: '20'
+          cache: 'npm'
+      - name: Install npm dependencies
+        run: npm ci
+      - name: Build CSS
+        run: npm run build-css
       - name: Setup Pages
         id: pages
         uses: actions/configure-pages@45bfe0192ca1faeb007ade9deae92b16b8254a0d # v6.0.0