feat: Phase 3 & 4 — monitoring, discovery, polish, deployment

Phase 3 — Discovery & Monitoring:
- Network scanner: nmap wrapper + mock fallback, fingerprint service (35 signatures)
- Status checker: ping/http/https/tcp/ssh/prometheus/health per-node checks
- APScheduler: status checks every 60s, WebSocket broadcast
- WebSocket /ws/status: live node status updates to frontend
- Sidebar panels: Pending Devices, Hidden Devices, Scan History
- Auth token persisted to localStorage (survive page refresh)
- 24 new backend tests (scan flow + status_checker)

Phase 4 — Polish & Deployment:
- Auto-layout: Dagre hierarchical TB via Toolbar button
- Export PNG: html-to-image download via Toolbar button
- Scan config modal: CIDR ranges + check interval, GET/POST /api/v1/scan/config
- Dockerfile.backend (Python 3.13 slim + nmap), Dockerfile.frontend (nginx)
- docker-compose.yml with data volume and NET_RAW cap for ping
- scripts/lxc-install.sh: Proxmox VE systemd bootstrap
- README.md: quick-start, config reference, stack overview

frontend/src/utils/export.ts

@@ -0,0 +1,20 @@
+import { toPng } from 'html-to-image'
+
+/**
+ * Export the React Flow canvas as a PNG and trigger a browser download.
+ * Pass the `.react-flow` wrapper element.
+ */
+export async function exportToPng(element: HTMLElement): Promise<void> {
+  const dataUrl = await toPng(element, {
+    backgroundColor: '#0d1117',
+    style: {
+      // Exclude controls and minimap from the export
+      '--xy-controls-display': 'none',
+    },
+  })
+
+  const link = document.createElement('a')
+  link.download = 'homelable-canvas.png'
+  link.href = dataUrl
+  link.click()
+}