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/App.tsx

@@ -115,6 +115,7 @@ const AdminAuditLog = lazy(() => import('./pages/AdminAuditLog'));
 const AdminLandings = lazy(() => import('./pages/AdminLandings'));
 const AdminLandingEditor = lazy(() => import('./pages/AdminLandingEditor'));
 const AdminLandingStats = lazy(() => import('./pages/AdminLandingStats'));
+const AdminReferralNetwork = lazy(() => import('./pages/ReferralNetwork'));
 
 function ProtectedRoute({
   children,
@@ -919,6 +920,16 @@ function App() {
             </PermissionRoute>
           }
         />
+        <Route
+          path="/admin/referral-network"
+          element={
+            <PermissionRoute permission="stats:read">
+              <LazyPage>
+                <AdminReferralNetwork />
+              </LazyPage>
+            </PermissionRoute>
+          }
+        />
         <Route
           path="/admin/payment-methods"
           element={