Build social graph for gist reaction networks

## Description
Visualize connections between wallets based on shared reactions.

## Requirements
- Force-directed graph layout
- Node size by activity level
- Edge weight by shared reactions
- Cluster detection
- Export graph data

## Files to Touch
- analytics/components/charts/SocialGraph.tsx
- analytics/lib/graph-builder.ts
- analytics/package.json

**Complexity: 300 points**