feat(billing): add useBillingStore and /billing/state integration

T32: Single frontend source of truth for subscription / plan / feature
state. New Zustand `useBillingStore` fetches `/billing/state` (auto-fetch
on login via authStore, reset on logout), exposes `refetch` for
post-Checkout refresh, and is supported by a `useBillingPoll` hook
that re-fetches every 60s while authenticated. The new `billingApi`
client transforms the snake_case backend payload to camelCase at a
single boundary so the rest of the frontend never sees `plan_billing`
or `enabled_features`.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/api/index.ts

@@ -9,6 +9,7 @@ export { default as foldersApi } from './folders'
 export { default as stepsApi } from './steps'
 export { default as stepCategoriesApi } from './stepCategories'
 export { default as accountsApi } from './accounts'
+export { default as billingApi } from './billing'
 export { default as adminApi } from './admin'
 export { treeMarkdownApi } from './treeMarkdown'
 export { default as analyticsApi } from './analytics'