Add calculation docs and redesign UI with better components ...

- Added docs/calculations.md explaining formula, Base, royalties, auto-calc
- Redesigned UI using Cards, Metric Cards, Alerts, Badges, Page Header
- Fixed icon buttons using `with-icon` class
- Replaced description-list summary with metric cards grid
- Added accent styling for total S metric card
- Updated message display to use alert components
- Fixed duplicate export button IDs

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Initial implementation of fire show calculator ...

- Added gnexus-ui-kit as git submodule
- Created index.html with calculator UI
- Implemented formula solver for one unknown variable
- Added artist roster with role assignment
- Calculated royalties based on net amount after material costs
- Added localStorage persistence and JSON export/import
- Created docs/plan.md with project documentation

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>