Fix TypeScript build errors

- Remove unused variables (allFolders, getFolderDepth, hasChildren, legacyCategories)
- Fix Lucide icon title prop by wrapping in span elements

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/components/tree-editor/TreeMetadataForm.tsx

@@ -24,7 +24,6 @@ export function TreeMetadataForm() {
   } = useTreeEditorStore()
 
   const [categories, setCategories] = useState<CategoryListItem[]>([])
-  const [legacyCategories, setLegacyCategories] = useState<string[]>([])
   const [customCategory, setCustomCategory] = useState(false)
 
   // Load categories