fix: make trial, campaign, and no-subscription node colors distinct

- campaignUser: #4dd9c0 (teal) → #fbbf24 (amber) — no longer blends with trial blue
- regular: #6b7280 (gray) → #94a3b8 (slate) — visible on dark background
- Campaign edges updated to amber tint
- defaultNodeColor now references NODE_COLORS.regular instead of hardcoded value
- Legend gradient uses actual campaign palette colors

src/pages/ReferralNetwork/utils.ts

@@ -18,8 +18,8 @@ export const NODE_COLORS = {
   trialActive: '#38bdf8',
   paidExpired: '#ef4444',
   trialExpired: '#fb923c',
-  campaignUser: '#4dd9c0',
-  regular: '#6b7280',
+  campaignUser: '#fbbf24',
+  regular: '#94a3b8',
 } as const;
 
 /**