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/components/library/AddToFolderMenu.tsx

@@ -1,6 +1,6 @@
 import { useState, useEffect, useRef } from 'react'
 import { FolderPlus, Check, Plus } from 'lucide-react'
-import { foldersApi } from '@/api'
+import { foldersApi } from '@/api/folders'
 import type { FolderListItem } from '@/types'
 import { cn } from '@/lib/utils'