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>
+  );
+}

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>
+  );
+}

src/lib/admin-queries.ts

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+import { db } from "@/db";
+import { clients, payments } from "@/db/schema";
+import { eq } from "drizzle-orm";
+
+export type ClientWithPayments = {
+  id: string;
+  name: string;
+  brand_name: string;
+  token: string;
+  accepted_total: string;
+  created_at: Date;
+  payments: Array<{
+    id: string;
+    label: string;
+    status: string;
+    amount: string;
+  }>;
+};
+
+export async function getAllClientsWithPayments(): Promise<ClientWithPayments[]> {
+  const allClients = await db
+    .select()
+    .from(clients)
+    .orderBy(clients.created_at);
+
+  if (allClients.length === 0) return [];
+
+  const allPayments = await db
+    .select()
+    .from(payments);
+
+  return allClients.map((c) => ({
+    id: c.id,
+    name: c.name,
+    brand_name: c.brand_name,
+    token: c.token,
+    accepted_total: c.accepted_total ?? "0",
+    created_at: c.created_at,
+    payments: allPayments
+      .filter((p) => p.client_id === c.id)
+      .map((p) => ({
+        id: p.id,
+        label: p.label,
+        status: p.status,
+        amount: p.amount,
+      })),
+  }));
+}
+
+export async function getClientById(id: string) {
+  const rows = await db
+    .select()
+    .from(clients)
+    .where(eq(clients.id, id))
+    .limit(1);
+  return rows[0] ?? null;
+}