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/store/authStore.ts

@@ -1,8 +1,8 @@
 import { create } from 'zustand'
 import { persist } from 'zustand/middleware'
 import type { User, Token, UserCreate, UserLogin, Account, SubscriptionDetails } from '@/types'
-import { authApi } from '@/api'
-import { apiClient } from '@/api'
+import { authApi } from '@/api/auth'
+import { apiClient } from '@/api/client'
 
 interface AuthState {
   user: User | null