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/AccountSettingsPage.tsx

@@ -1,7 +1,7 @@
 import { useEffect, useState } from 'react'
 import { Link } from 'react-router-dom'
 import { Building2, Users, Mail, Crown, Loader2, AlertCircle, Check, X, Settings, FolderTree } from 'lucide-react'
-import { accountsApi } from '@/api'
+import { accountsApi } from '@/api/accounts'
 import type { Account, AccountMember, AccountInvite } from '@/types'
 import { cn } from '@/lib/utils'
 import { usePermissions } from '@/hooks/usePermissions'