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>

backend/app/schemas/network_diagram.py

@@ -83,6 +83,7 @@ class NetworkDiagramListItem(BaseModel):
     client_name: str | None = None
     description: str | None = None
     node_count: int = 0
+    category_counts: dict[str, int] = Field(default_factory=dict)
     created_by: UUID | None = None
     created_at: datetime
     updated_at: datetime