feat(02-02): add admin client list page and create-client flow

- /admin page: Server Component fetching all clients with payment badges
- ClientRow component with Acconto/Saldo status badges and secret link
- /admin/clients/new: form wired to createClient Server Action
- createClient action: Zod validation, inserts client + 2 payment stubs (Acconto 50%, Saldo 50%)
- Token auto-generated server-side via nanoid $defaultFn
- Redirects to /admin/clients/[id] after creation; revalidates /admin

src/app/admin/clients/new/page.tsx

@@ -0,0 +1,69 @@
+import { createClient } from "./actions";
+import { Button } from "@/components/ui/button";
+import { Input } from "@/components/ui/input";
+import { Label } from "@/components/ui/label";
+import { Textarea } from "@/components/ui/textarea";
+import {
+  Card,
+  CardContent,
+  CardHeader,
+  CardTitle,
+} from "@/components/ui/card";
+import Link from "next/link";
+
+export default function NewClientPage() {
+  return (
+    <div className="max-w-xl">
+      <div className="mb-6">
+        <Link href="/admin" className="text-sm text-gray-500 hover:text-gray-700">
+          ← Clienti
+        </Link>
+      </div>
+      <Card>
+        <CardHeader>
+          <CardTitle>Nuovo cliente</CardTitle>
+        </CardHeader>
+        <CardContent>
+          <form action={createClient} className="space-y-4">
+            <div className="space-y-1">
+              <Label htmlFor="name">Nome cliente</Label>
+              <Input
+                id="name"
+                name="name"
+                type="text"
+                placeholder="es. Marco Rossi"
+                required
+              />
+            </div>
+            <div className="space-y-1">
+              <Label htmlFor="brand_name">Nome brand</Label>
+              <Input
+                id="brand_name"
+                name="brand_name"
+                type="text"
+                placeholder="es. Rossi Studio"
+                required
+              />
+            </div>
+            <div className="space-y-1">
+              <Label htmlFor="brief">Brief del progetto</Label>
+              <Textarea
+                id="brief"
+                name="brief"
+                placeholder="Descrizione del progetto e degli obiettivi..."
+                rows={5}
+                required
+              />
+            </div>
+            <div className="flex gap-3 pt-2">
+              <Button type="submit">Crea cliente</Button>
+              <Button variant="outline" asChild>
+                <Link href="/admin">Annulla</Link>
+              </Button>
+            </div>
+          </form>
+        </CardContent>
+      </Card>
+    </div>
+  );
+}