chore: add GitHub Pages deployment workflow for demo

[goodlime]

Summary

• Adds .github/workflows/pages.yml to deploy the demo Angular project to GitHub Pages on push to main
• Builds with --base-href /grapesjs-angular/ and publishes dist/demo/browser/ via actions/deploy-pages@v4
• Uses Node 24 to match ci.yml

Required before first run

GitHub Pages must be enabled in repo settings: Settings → Pages → Source: GitHub Actions. Without this, deploy-pages@v4 fails on the github-pages environment.

Target URL

https://internetliquid.github.io/grapesjs-angular/

Test plan

• Enable Pages (Source: GitHub Actions) in repo settings
• Merge PR and watch "Deploy Demo to GitHub Pages" workflow on main
• Both build and deploy jobs pass
• Site loads at target URL and GrapesJS editor renders
• DevTools Network tab confirms assets load under /grapesjs-angular/… prefix (no 404s)