feat: update frontend for account-based subscriptions

Replace all team_id/team_admin references with account_id/owner across
types, store, hooks, API clients, components, and pages. Add new
AccountSettingsPage, UpgradePrompt, CheckoutButton, useSubscription
hook, and accounts API client. AuthStore now parallel-fetches account
and subscription data alongside user profile.

Also fix folder sidebar not refreshing after tree deletion by
dispatching the folder-changed event in handleDeleteTree.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/api/index.ts

@@ -8,3 +8,4 @@ export { default as categoriesApi } from './categories'
 export { default as foldersApi } from './folders'
 export { default as stepsApi } from './steps'
 export { default as stepCategoriesApi } from './stepCategories'
+export { default as accountsApi } from './accounts'