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/utils/__tests__/brandIcons.test.ts

@@ -0,0 +1,57 @@
+import { describe, it, expect } from 'vitest'
+import {
+  BRAND_ICON_PREFIX,
+  isBrandIconKey,
+  brandIconSlug,
+  brandIconUrl,
+  resolveCustomIcon,
+  ICON_MAP,
+} from '../nodeIcons'
+
+describe('brand icon helpers', () => {
+  it('isBrandIconKey returns true only for prefixed keys', () => {
+    expect(isBrandIconKey('brand:plex')).toBe(true)
+    expect(isBrandIconKey('plex')).toBe(false)
+    expect(isBrandIconKey('plug')).toBe(false)
+    expect(isBrandIconKey(undefined)).toBe(false)
+    expect(isBrandIconKey(null)).toBe(false)
+    expect(isBrandIconKey('')).toBe(false)
+  })
+
+  it('brandIconSlug strips the prefix', () => {
+    expect(brandIconSlug(`${BRAND_ICON_PREFIX}home-assistant`)).toBe('home-assistant')
+  })
+
+  it('brandIconUrl points at jsDelivr CDN', () => {
+    expect(brandIconUrl('plex')).toBe(
+      'https://cdn.jsdelivr.net/gh/homarr-labs/dashboard-icons/svg/plex.svg',
+    )
+  })
+})
+
+describe('resolveCustomIcon', () => {
+  it('returns null when no key', () => {
+    expect(resolveCustomIcon(undefined)).toBeNull()
+    expect(resolveCustomIcon('')).toBeNull()
+  })
+
+  it('resolves legacy lucide keys', () => {
+    const r = resolveCustomIcon('plug')
+    expect(r?.kind).toBe('lucide')
+    if (r?.kind === 'lucide') expect(r.icon).toBe(ICON_MAP['plug'])
+  })
+
+  it('resolves brand-prefixed keys to a CDN url', () => {
+    const r = resolveCustomIcon('brand:plex')
+    expect(r?.kind).toBe('brand')
+    if (r?.kind === 'brand') {
+      expect(r.slug).toBe('plex')
+      expect(r.url).toContain('cdn.jsdelivr.net')
+      expect(r.url).toContain('/plex.svg')
+    }
+  })
+
+  it('returns null for unknown legacy key', () => {
+    expect(resolveCustomIcon('definitely-not-a-known-icon-key')).toBeNull()
+  })
+})