feat(phase-27): add advanced ATT&CK Navigator-style heatmap with layers, filters and export (T-221 to T-223)

frontend/src/components/Sidebar.tsx

@@ -15,6 +15,7 @@ import {
   Database,
   Crosshair,
   Zap,
+  Grid3X3,
 } from "lucide-react";
 import { useAuth } from "../context/AuthContext";
 
@@ -28,6 +29,7 @@ interface NavItem {
 const mainLinks: NavItem[] = [
   { to: "/dashboard", label: "Dashboard", icon: LayoutDashboard },
   { to: "/techniques", label: "ATT&CK Matrix", icon: Shield },
+  { to: "/matrix", label: "Advanced Heatmap", icon: Grid3X3 },
   {
     to: "/tests",
     label: "Tests",