Add layout y dashboard del admin

mvp/b2c/src/app/admin/layout.tsx

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+import Link from 'next/link';
+import type { Metadata } from 'next';
+import { requireAdmin } from '@/lib/auth/current-user';
+
+export const metadata: Metadata = { title: 'Admin · Reformix' };
+
+export default async function AdminLayout({ children }: { children: React.ReactNode }) {
+  await requireAdmin();
+  return (
+    <div className="min-h-screen bg-gray-50">
+      <header className="sticky top-0 z-10 bg-white border-b border-gray-200">
+        <div className="max-w-6xl mx-auto px-6 h-16 flex items-center justify-between">
+          <Link href="/admin" className="flex items-center gap-3">
+            <span className="inline-flex items-center justify-center w-8 h-8 rounded-lg bg-black text-white font-black italic text-lg leading-none">R</span>
+            <span className="font-extrabold tracking-tight text-black">Reformix</span>
+            <span className="text-gray-300">/</span>
+            <span className="text-sm font-medium text-gray-600">Admin</span>
+          </Link>
+          <nav className="flex items-center gap-4 text-xs font-medium">
+            <Link href="/admin" className="text-gray-500 hover:text-black">Resumen</Link>
+            <Link href="/admin/usuarios" className="text-gray-500 hover:text-black">Usuarios</Link>
+            <Link href="/admin/planes" className="text-gray-500 hover:text-black">Planes</Link>
+            <form action="/logout" method="post"><button type="submit" className="text-gray-500 hover:text-black">Salir</button></form>
+          </nav>
+        </div>
+      </header>
+      <main className="max-w-6xl mx-auto px-6 py-8">{children}</main>
+    </div>
+  );
+}

mvp/b2c/src/app/admin/page.tsx

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+import { listTenants, listUsers, listPlans } from '@/db/admin-queries';
+
+export const dynamic = 'force-dynamic';
+
+export default async function AdminHome() {
+  const [tenants, users, plans] = await Promise.all([listTenants(), listUsers(), listPlans()]);
+  const cards = [
+    { label: 'Reformistas (tenants)', value: tenants.length },
+    { label: 'Usuarios', value: users.length },
+    { label: 'Planes activos', value: plans.length },
+    { label: 'En trial', value: tenants.filter((t) => t.subscriptionStatus === 'trial').length },
+  ];
+  return (
+    <div className="flex flex-col gap-6">
+      <h1 className="text-2xl font-black tracking-tight text-black">Resumen</h1>
+      <div className="grid grid-cols-2 md:grid-cols-4 gap-4">
+        {cards.map((c) => (
+          <div key={c.label} className="bg-white border border-gray-200 rounded-xl p-5">
+            <div className="text-3xl font-black text-black">{c.value}</div>
+            <div className="text-xs text-gray-500 mt-1">{c.label}</div>
+          </div>
+        ))}
+      </div>
+    </div>
+  );
+}