Fix Heroku runtime error by using static file server

- Replace 'ember serve' with 'serve' for production
- 'ember serve' requires ember-cli (devDependency, not in production)
- Add 'serve' package to serve built static files from dist/
- Update build script to use --environment=production
- Add 'dev' script for local development with ember serve
- serve uses PORT env var from Heroku, falls back to 4300

This fixes the "ember: not found" error on Heroku.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Author: compwron