feat: add admin traffic usage page with TanStack Table

Add AdminTrafficUsage page showing per-user traffic statistics by node
with period filtering (1/3/7/14/30d), server-side sorting, search,
pagination, and CSV export. Uses @tanstack/react-table with manual
sorting and dynamic node columns.

src/pages/AdminPanel.tsx

@@ -216,6 +216,16 @@ const EnvelopeIcon = () => (
   </svg>
 );
 
+const ArrowsUpDownIcon = () => (
+  <svg className="h-5 w-5" fill="none" viewBox="0 0 24 24" stroke="currentColor" strokeWidth={1.5}>
+    <path
+      strokeLinecap="round"
+      strokeLinejoin="round"
+      d="M3 7.5L7.5 3m0 0L12 7.5M7.5 3v13.5m13.5 0L16.5 21m0 0L12 16.5m4.5 4.5V7.5"
+    />
+  </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" />
@@ -317,6 +327,12 @@ export default function AdminPanel() {
           title: t('admin.nav.payments'),
           description: t('admin.panel.paymentsDesc'),
         },
+        {
+          to: '/admin/traffic-usage',
+          icon: <ArrowsUpDownIcon />,
+          title: t('admin.nav.trafficUsage'),
+          description: t('admin.panel.trafficUsageDesc'),
+        },
       ],
     },
     {