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/api/admin/dashboard/route.js

@@ -0,0 +1,15 @@
+import { NextResponse } from 'next/server';
+import { requireUser } from '@/lib/session';
+
+const ENGINE_URL    = process.env.ENGINE_URL    || 'http://127.0.0.1:3030';
+const ENGINE_SECRET = process.env.ENGINE_SECRET || '';
+
+export async function GET() {
+  const user = await requireUser();
+  if (!user?.isAdmin) return NextResponse.json({ error: 'Forbidden' }, { status: 403 });
+  const res = await fetch(`${ENGINE_URL}/api/admin/dashboard`, {
+    headers: { 'x-internal-secret': ENGINE_SECRET, 'x-user-id': String(user.id) },
+    cache: 'no-store',
+  });
+  return NextResponse.json(await res.json());
+}