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/app/admin/layout.tsx

@@ -0,0 +1,14 @@
+import { NavBar } from "@/components/admin/NavBar";
+
+export default function AdminLayout({
+  children,
+}: {
+  children: React.ReactNode;
+}) {
+  return (
+    <div className="min-h-screen bg-gray-50">
+      <NavBar />
+      <main className="max-w-5xl mx-auto px-6 py-8">{children}</main>
+    </div>
+  );
+}