feat: attack chain DAG visualization, report export, and UI polish

- Add interactive attack chain DAG using vis-network with hierarchical
  layout, status-colored nodes, risk-based sizing, and click handlers
- Add pentest session export API (GET /sessions/:id/export) supporting
  both JSON and Markdown report formats
- Redesign attack chain tab with graph/list toggle views
- Add export buttons (MD/JSON) to session header with Blob download
- Show exploitable badge and endpoint on finding cards
- Add export_pentest_report server function for dashboard

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

compliance-dashboard/src/app.rs

@@ -53,6 +53,7 @@ const MAIN_CSS: Asset = asset!("/assets/main.css");
 const TAILWIND_CSS: Asset = asset!("/assets/tailwind.css");
 const VIS_NETWORK_JS: Asset = asset!("/assets/vis-network.min.js");
 const GRAPH_VIZ_JS: Asset = asset!("/assets/graph-viz.js");
+const ATTACK_CHAIN_VIZ_JS: Asset = asset!("/assets/attack-chain-viz.js");
 
 #[component]
 pub fn App() -> Element {
@@ -62,6 +63,7 @@ pub fn App() -> Element {
         document::Link { rel: "stylesheet", href: MAIN_CSS }
         document::Script { src: VIS_NETWORK_JS }
         document::Script { src: GRAPH_VIZ_JS }
+        document::Script { src: ATTACK_CHAIN_VIZ_JS }
         Router::<Route> {}
     }
 }