feat: chat izq/der en tickets, correos con ultimo mensaje, dashboard filtrable solo admin/supervisor

frontend/src/components/Sidebar.tsx

@@ -9,7 +9,11 @@ export default function Sidebar() {
   const { user, logout } = useAuth();
 
   const navItems = [
-    { href: "/", icon: "bi-speedometer2", label: "Dashboard" },
+    // Dashboard solo para administradores (todas las áreas) y
+    // supervisores (solo su área).
+    ...((user?.role === "admin" || user?.role === "supervisor")
+      ? [{ href: "/", icon: "bi-speedometer2", label: "Dashboard" }]
+      : []),
     { href: "/tickets", icon: "bi-ticket-detailed", label: "Tickets" },
     { href: "/knowledge-base", icon: "bi-journal-bookmark", label: "Knowledge Base" },
   ];