feat: network diagrams UX overhaul — icons, empty canvas, properties panel

- Colorize: semantic category colors for all device types (network=blue,
  security=orange, compute=emerald, endpoint=amber, storage=violet,
  cloud=cyan, infra=steel); better icons (Router, ShieldAlert, Boxes,
  Package, Gauge, PlugZap, Video, Radio); MiniMap uses category colors
- Onboard: centered AI generate prompt on empty canvas with 5 MSP-specific
  example chips, ⌘↵ shortcut, spinner; AIAssistPanel only shown with nodes
- Arrange: properties panel — status badge grid at top, fields grouped into
  Network (IP/Subnet/VLAN) and Hardware (Hostname/Vendor/Model/Role) sections
- Delight: segmented topology color bar on listing cards; backend returns
  category_counts via single extra query on list endpoint
- Harden: real PNG export via html-to-image + getNodesBounds/getViewportForBounds
- Polish: ChevronDown replaces unicode ▾, click-outside for client filter,
  consistent spinner in empty prompt

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

frontend/package-lock.json

@@ -23,6 +23,7 @@
         "class-variance-authority": "^0.7.1",
         "clsx": "^2.1.1",
         "date-fns": "^4.1.0",
+        "html-to-image": "^1.11.13",
         "immer": "^11.1.3",
         "lucide-react": "^0.563.0",
         "monaco-editor": "^0.55.1",
@@ -5331,6 +5332,12 @@
       "dev": true,
       "license": "MIT"
     },
+    "node_modules/html-to-image": {
+      "version": "1.11.13",
+      "resolved": "https://registry.npmjs.org/html-to-image/-/html-to-image-1.11.13.tgz",
+      "integrity": "sha512-cuOPoI7WApyhBElTTb9oqsawRvZ0rHhaHwghRLlTuffoD1B2aDemlCruLeZrUIIdvG7gs9xeELEPm6PhuASqrg==",
+      "license": "MIT"
+    },
     "node_modules/html-url-attributes": {
       "version": "3.0.1",
       "resolved": "https://registry.npmjs.org/html-url-attributes/-/html-url-attributes-3.0.1.tgz",