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/TreeEditorPage.tsx

@@ -3,7 +3,8 @@ import { useParams, useNavigate, useBlocker } from 'react-router-dom'
 import { useStore } from 'zustand'
 import { Undo2, Redo2, Save, CheckCircle2, Monitor, FileText, Code2, LayoutList } from 'lucide-react'
 import { getMonacoEditor } from '@/components/tree-editor/code-mode'
-import { treesApi, treeMarkdownApi } from '@/api'
+import { treesApi } from '@/api/trees'
+import { treeMarkdownApi } from '@/api/treeMarkdown'
 import type { TreeCreate, TreeUpdate, TreeStatus } from '@/types'
 import { useTreeEditorStore, useTreeEditorTemporal } from '@/store/treeEditorStore'
 import { TreeEditorLayout } from '@/components/tree-editor/TreeEditorLayout'