refactor: extract admin UI to static files (#395)
- Move HTML/CSS/JS from inline const to static/admin.{html,css,js}
- Serve static files via tower-http ServeDir
- Add Io error variant for file read errors
- Update Dockerfile to copy static directory
admin.rs: 730 → 310 lines
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-authored-by: Claude <noreply@anthropic.com>
authored by