fix: remove campaignUser fill color — campaign membership shown via edges

Campaign users no longer get a separate amber fill color. Their campaign
association is already visible through the edge connecting them to the
campaign node. Users from campaigns now show their subscription status
color (or regular gray if no subscription), reducing visual noise.

src/pages/ReferralNetwork/components/NetworkGraph.tsx

@@ -166,7 +166,6 @@ function buildFullGraph(graphData: NetworkGraphData): Graph {
   graphData.users.forEach((user) => {
     const fillColor = getNodeFillColor(
       user.subscription_status,
-      user.campaign_id,
       user.direct_referrals,
       user.is_partner,
     );