fix(icons): narrow ICON_MAP lookup type for strict build

tsc -b (used in npm run build) flagged TS2774 because LucideIcon is a
function and therefore always truthy. Cast the lookup result to
LucideIcon | undefined so the falsy branch becomes meaningful.

frontend/src/utils/nodeIcons.ts

@@ -218,6 +218,6 @@ export function resolveCustomIcon(customIconKey?: string): ResolvedIcon | null {
     const slug = brandIconSlug(customIconKey)
     return { kind: 'brand', slug, url: brandIconUrl(slug) }
   }
-  const icon = ICON_MAP[customIconKey]
+  const icon = ICON_MAP[customIconKey] as LucideIcon | undefined
   return icon ? { kind: 'lucide', icon } : null
 }