refactor: replace barrel imports with direct module imports for tree-shaking

Replace all `from '@/api'` barrel imports with direct imports from
specific module files (e.g. `from '@/api/trees'`) across 20 files.
This enables better tree-shaking so each page only bundles the API
modules it actually uses.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/pages/TreeLibraryPage.tsx

@@ -1,7 +1,9 @@
 import { useEffect, useState, useCallback } from 'react'
 import { useNavigate, Link } from 'react-router-dom'
 import { Plus, X, FolderOpen } from 'lucide-react'
-import { treesApi, categoriesApi, foldersApi } from '@/api'
+import { treesApi } from '@/api/trees'
+import { categoriesApi } from '@/api/categories'
+import { foldersApi } from '@/api/folders'
 import type { TreeListItem, CategoryListItem, FolderListItem } from '@/types'
 import { FolderSidebar } from '@/components/library/FolderSidebar'
 import { FolderEditModal } from '@/components/library/FolderEditModal'