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/AdminPanel.tsx

@@ -248,6 +248,17 @@ const ArrowsUpDownIcon = () => (
   </svg>
 );
 
+const NetworkGraphIcon = () => (
+  <svg className="h-5 w-5" fill="none" viewBox="0 0 24 24" stroke="currentColor" strokeWidth={1.5}>
+    <path
+      strokeLinecap="round"
+      strokeLinejoin="round"
+      d="M7.5 21L3 16.5m0 0L7.5 12M3 16.5h13.5m0-13.5L21 7.5m0 0L16.5 12M21 7.5H7.5"
+    />
+    <circle cx="12" cy="12" r="2" fill="currentColor" stroke="none" />
+  </svg>
+);
+
 const ChevronRightIcon = () => (
   <svg className="h-4 w-4" fill="none" viewBox="0 0 24 24" stroke="currentColor" strokeWidth={2}>
     <path strokeLinecap="round" strokeLinejoin="round" d="M8.25 4.5l7.5 7.5-7.5 7.5" />
@@ -569,6 +580,13 @@ export default function AdminPanel() {
           description: t('admin.panel.withdrawalsDesc'),
           permission: 'withdrawals:read',
         },
+        {
+          to: '/admin/referral-network',
+          icon: <NetworkGraphIcon />,
+          title: t('admin.nav.referralNetwork'),
+          description: t('admin.panel.referralNetworkDesc'),
+          permission: 'stats:read',
+        },
       ],
     },
     {