Add WebContainer-based preview deployment system

[dimitrieh]

Summary

• Adds a GitHub Actions workflow that deploys branch previews to Netlify using WebContainer
• Each branch gets its own preview URL where Node-RED boots and runs entirely in the browser
• Deploys surface as GitHub Deployment objects in PRs (no comment spam)

How it works

1. On push, the workflow snapshots the source code using @webcontainer/snapshot
2. Generates a single-page HTML loader with a progress UI (boot → install → build → start)
3. Deploys to Netlify with branch-specific aliases
4. The browser-side loader boots a WebContainer, mounts the snapshot, runs npm ci && npm run build && npm start, and loads the Node-RED editor in an iframe

Test plan

• Push a branch and verify the GitHub Actions workflow succeeds
• Verify the deployment URL loads the WebContainer preview
• Verify the GitHub Deployment status appears in PRs
• Verify the Node-RED editor loads successfully in the iframe