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/MyTreesPage.tsx

@@ -1,7 +1,8 @@
 import { useEffect, useState } from 'react'
 import { useNavigate, Link } from 'react-router-dom'
 import { Play, Pencil, Share2, Trash2, GitBranch, Clock, TrendingUp, FolderTree } from 'lucide-react'
-import { treesApi, sessionsApi } from '@/api'
+import { treesApi } from '@/api/trees'
+import { sessionsApi } from '@/api/sessions'
 import type { TreeListItem } from '@/types'
 import { TagBadges } from '@/components/common/TagBadges'
 import { ConfirmDialog } from '@/components/common/ConfirmDialog'