fix(docs): add cleanUrls to Vercel config for static export routing

Static export generates embed/theme-preview.html but Vercel was
returning 404 for /embed/theme-preview without the .html extension.

Co-Authored-By: Claude Sonnet 4.6 <[email protected]>

Author: 3o14

.github/workflows/deploy-docs.yml

@@ -44,7 +44,7 @@ jobs:
       - name: Package static output for Vercel
         run: |
           mkdir -p docs/.vercel/output/static
-          echo '{"version":3}' > docs/.vercel/output/config.json
+          echo '{"version":3,"cleanUrls":true}' > docs/.vercel/output/config.json
           cp -r docs/out/. docs/.vercel/output/static/
 
       - name: Deploy to Vercel