Make demo site Vercel-deployable

[anth-volk]

Fixes #9

Summary

• Set root: 'demo' in vite.demo.config.ts so Vite finds index.html
• Set build.outDir to demo-dist/ to avoid conflicting with library dist/
• Dev server and production build both verified locally

Vercel settings

• Build command: vite build --config vite.demo.config.ts
• Output directory: demo-dist

Test plan

• bun run dev:demo still works locally
• npx vite build --config vite.demo.config.ts produces demo-dist/ with working HTML
• Deploy to Vercel with the settings above

🤖 Generated with Claude Code