feat: admin dashboard UI — DashboardSection as default panel

AdminPanel: Сводка раздел первый (initialSection='dashboard')
DashboardSection: users stats, channels by platform, posts stats,
  revenue vs AI costs cards, drafts pending alert, registrations bar chart 14d
SECTIONS: +Dashboard, +Engine (Движок)
API route: /api/admin/dashboard proxy

app/system/page.js

@@ -13,7 +13,7 @@ export default async function SystemPage({ searchParams }) {
   return (
     <>
       <Header user={user} />
-      <AdminPanel initialSection={searchParams?.section || 'settings'} />
+      <AdminPanel initialSection={searchParams?.section || 'dashboard'} />
     </>
   );
 }