Create the basic web server module:
- Omni/Jr/Web.hs with Servant API type and Warp runner
- Add 'jr web [--port=PORT]' command to Jr.hs docopt and move handler
- GET / returns simple HTML with 'Jr Web UI' heading
- Use patterns from Biz/Dragons.hs (Servant + Warp + Lucid)
- Default port 8080, configurable via --port flag