Build gist radius heat ring visualization

## Description
Concentric ring visualization showing gist density at different radii.

## Requirements
- Ring chart from center point
- Density at 100m, 250m, 500m, 1km, 5km rings
- Interactive center point selection
- Density comparison rings
- Export ring data

## Files to Touch
- analytics/components/charts/RadiusRings.tsx

**Complexity: 200 points**