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/SessionDetailPage.tsx

@@ -1,7 +1,8 @@
 import { useEffect, useState } from 'react'
 import { useParams, useNavigate } from 'react-router-dom'
 import { Copy, Check, Eye, Save } from 'lucide-react'
-import { sessionsApi, stepsApi } from '@/api'
+import { sessionsApi } from '@/api/sessions'
+import { stepsApi } from '@/api/steps'
 import { ExportPreviewModal } from '@/components/session/ExportPreviewModal'
 import { SaveSessionAsTreeModal } from '@/components/session/SaveSessionAsTreeModal'
 import { StepRatingModal } from '@/components/session/StepRatingModal'