feat: add referral network graph visualization page

Interactive Sigma.js + graphology graph with ForceAtlas2 web worker
layout, campaign/user node coloring, filters via nodeReducer pattern,
search highlighting, user/campaign detail side panels, and 4-locale
translations. Dependencies: sigma v3, graphology, graphology-layout-forceatlas2.

src/pages/ReferralNetwork/index.tsx

@@ -0,0 +1,3 @@
+import { ReferralNetwork } from './ReferralNetwork';
+
+export default ReferralNetwork;