feat: dashboard box "Messaggi Clienti" — chat clienti in attesa di risposta

Nuovo MessagesWidget nella colonna 1/3 della dashboard: riusa
getConversations() filtrando le conversazioni unread, mostra le prime 4
con anteprima e deep-link "Rispondi" a /admin/conversazioni?c=<clientId>.
Pill emerald "N Nuovi" + pallino pulsante, empty state quando non c'è nulla
in attesa. Stessa fonte di verità (clients.admin_last_read_at) del badge
sidebar. Aggiornato DESIGN-SYSTEM.md (inventory + note Conversazioni/dashboard)
e il mock design-reference/pagina-dashboard.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/admin/page.tsx

@@ -1,6 +1,7 @@
 import { Suspense } from "react";
 import { getDashboardStats } from "@/lib/dashboard-queries";
 import { FollowUpWidget } from "@/components/admin/dashboard/FollowUpWidget";
+import { MessagesWidget } from "@/components/admin/dashboard/MessagesWidget";
 import {
   getAnalyticsByYear,
   getTotalTrackedHours,
@@ -119,6 +120,11 @@ export default async function AdminDashboard({
           >
             <FollowUpWidget />
           </Suspense>
+          <Suspense
+            fallback={<div className="animate-pulse h-40 bg-muted rounded-xl" />}
+          >
+            <MessagesWidget />
+          </Suspense>
           <OffersSoldChart data={offersSold} />
         </div>
       </div>