fix: upgrade Node.js to v20 in GitHub Pages workflow

The deploy-github-pages.yml workflow was still using Node.js 18, which is incompatible with Vite 7 (requires 20.19+ or 22.12+). This was causing build failures with the error "crypto.hash is not a function".

Updated to match the Node.js version used in the other deployment workflows (PR preview and production).

Author: claude

.github/workflows/deploy-github-pages.yml

@@ -32,7 +32,7 @@ jobs:
       - name: Setup Node.js
         uses: actions/setup-node@v4
         with:
-          node-version: '18'
+          node-version: '20'
           cache: 'npm'
 
       - name: Setup Pages