feat(icons): add brand icon picker from dashboard-icons

Add a second tab in the node Icon picker to choose from the ~2250 brand
icons hosted by homarr-labs/dashboard-icons (Plex, Sonarr, Home
Assistant, etc.) served via jsDelivr CDN. The Generic tab keeps the
existing lucide picker unchanged.

Storage uses a 'brand:<slug>' prefix on custom_icon, so existing nodes
referencing lucide keys keep working with zero migration. A new
resolveCustomIcon helper returns a discriminated union (lucide | brand)
and a NodeIcon component centralizes rendering for both kinds.

Includes a manifest fetch script (scripts/fetch-dashboard-icons.mjs)
and a checked-in dashboardIcons.json snapshot.

frontend/src/components/canvas/nodes/ProxmoxGroupNode.tsx

@@ -3,7 +3,8 @@ import { Handle, Position, NodeResizer, type NodeProps, type Node } from '@xyflo
 import { Layers } from 'lucide-react'
 import type { NodeData } from '@/types'
 import { resolveNodeColors } from '@/utils/nodeColors'
-import { resolveNodeIcon } from '@/utils/nodeIcons'
+import { resolveNodeIcon, isBrandIconKey } from '@/utils/nodeIcons'
+import { NodeIcon } from '@/components/ui/NodeIcon'
 import { resolvePropertyIcon } from '@/utils/propertyIcons'
 import { useCanvasStore } from '@/stores/canvasStore'
 import { maskIp, splitIps } from '@/utils/maskIp'
@@ -87,7 +88,9 @@ export function ProxmoxGroupNode(props: NodeProps<Node<NodeData>>) {
               background: theme.colors.nodeIconBackground,
             }}
           >
-            {createElement(resolvedIcon, { size: 12 })}
+            {isBrandIconKey(data.custom_icon)
+              ? <NodeIcon typeIcon={Layers} customIconKey={data.custom_icon} size={12} />
+              : createElement(resolvedIcon, { size: 12 })}
           </div>
           <div className="flex flex-col min-w-0 flex-1">
             <span