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/TreeNavigationPage.tsx

@@ -1,6 +1,7 @@
 import { useEffect, useState } from 'react'
 import { useParams, useNavigate, useLocation } from 'react-router-dom'
-import { treesApi, sessionsApi } from '@/api'
+import { treesApi } from '@/api/trees'
+import { sessionsApi } from '@/api/sessions'
 import { useTreeNavigationShortcuts } from '@/hooks/useKeyboardShortcuts'
 import { useCustomStepFlow } from '@/hooks/useCustomStepFlow'
 import type { Tree, Session, DecisionRecord, TreeStructure } from '@/types'