feat(02-02): add admin-queries, NavBar, and admin layout

- src/lib/admin-queries.ts: getAllClientsWithPayments() and getClientById() for admin DB reads
- src/components/admin/NavBar.tsx: minimal nav with Clienti link and Esci (logout) button
- src/app/admin/layout.tsx: wraps all /admin/* pages with NavBar + centered main content area

src/components/admin/NavBar.tsx

@@ -0,0 +1,29 @@
+"use client";
+
+import Link from "next/link";
+import { signOut } from "next-auth/react";
+import { Button } from "@/components/ui/button";
+
+export function NavBar() {
+  return (
+    <nav className="border-b border-gray-200 bg-white px-6 py-3 flex items-center justify-between">
+      <div className="flex items-center gap-6">
+        <span className="font-semibold text-gray-900">ClientHub</span>
+        <Link
+          href="/admin"
+          className="text-sm text-gray-600 hover:text-gray-900 transition-colors"
+        >
+          Clienti
+        </Link>
+      </div>
+      <Button
+        variant="ghost"
+        size="sm"
+        onClick={() => signOut({ callbackUrl: "/admin/login" })}
+        className="text-sm text-gray-500"
+      >
+        Esci
+      </Button>
+    </nav>
+  );
+}