Redesign event registration form — single column, polished UI

- Remove 2-column grid (caused fields cramming side-by-side)
- All fields stack full-width for clean readability
- Radio/checkbox options in bordered card-style containers
- Proper focus states using site accent color
- Shadow and rounded inputs for depth
- Better section dividers with centered lines
- Submit button with icon, shadow, and loading spinner
- Error states with icon indicators
- Terms field highlighted with amber background
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>