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/step-library/StepDetailModal.tsx

@@ -2,7 +2,7 @@ import { useState, useEffect } from 'react'
 import { X, Star, Copy, Check, HelpCircle, Zap, CheckCircle, User, Calendar } from 'lucide-react'
 import { cn } from '@/lib/utils'
 import { MarkdownContent } from '@/components/ui/MarkdownContent'
-import { stepsApi } from '@/api'
+import { stepsApi } from '@/api/steps'
 import type { Step, Review } from '@/types/step'
 
 interface StepDetailModalProps {