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/BaseNode.tsx

@@ -3,7 +3,8 @@ import { Handle, Position, NodeResizer, useUpdateNodeInternals, useViewport, typ
 import { Cpu, MemoryStick, HardDrive, ExternalLink, type LucideIcon } 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 { useThemeStore } from '@/stores/themeStore'
 import { THEMES } from '@/utils/themes'
@@ -98,7 +99,9 @@ export function BaseNode({ id, data, selected, icon: typeIcon, width, height }:
             background: theme.colors.nodeIconBackground,
           }}
         >
-          {createElement(resolvedIcon, { size: 15 })}
+          {isBrandIconKey(data.custom_icon)
+            ? <NodeIcon typeIcon={typeIcon} customIconKey={data.custom_icon} size={15} />
+            : createElement(resolvedIcon, { size: 15 })}
         </div>
 
         {/* Label + IP */}