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'
 

frontend/src/components/library/FolderEditModal.tsx

@@ -1,6 +1,6 @@
 import { useState, useEffect, useMemo } from 'react'
 import { X } from 'lucide-react'
-import { foldersApi } from '@/api'
+import { foldersApi } from '@/api/folders'
 import type { FolderListItem, FolderCreate, FolderUpdate } from '@/types'
 import { cn } from '@/lib/utils'
 import { toast } from '@/lib/toast'

frontend/src/components/library/FolderSidebar.tsx

@@ -1,6 +1,6 @@
 import { useState, useEffect, useCallback } from 'react'
 import { Folder, ChevronDown, ChevronRight, Plus, MoreVertical, Pencil, Trash2, FolderPlus, X } from 'lucide-react'
-import { foldersApi } from '@/api'
+import { foldersApi } from '@/api/folders'
 import type { FolderListItem, FolderTreeItem } from '@/types'
 import { cn } from '@/lib/utils'
 

frontend/src/components/library/ShareTreeModal.tsx

@@ -1,7 +1,7 @@
 import { useState, useEffect } from 'react'
 import { X, Copy, Check, Link2, Users, Lock, Globe } from 'lucide-react'
 import type { TreeListItem, TreeShare, TreeVisibility } from '@/types'
-import { treesApi } from '@/api'
+import { treesApi } from '@/api/trees'
 import { cn } from '@/lib/utils'
 import { toast } from '@/lib/toast'