fix: upgrade CI Node.js to 20 for Vitest 4 compatibility

Vitest 4 requires Node >=20. The workflow was using Node 18,
causing test step to fail in CI.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: beihaili

.github/workflows/deploy.yml

@@ -29,7 +29,7 @@ jobs:
     - name: Setup Node.js 🟢
       uses: actions/setup-node@v4
       with:
-        node-version: '18'
+        node-version: '20'
         cache: 'npm'
 
     - name: Install Dependencies 📦