feat(billing): add useFeature, useFeatureLimit, useTrialBanner hooks

Phase 2 Task 33. Components can now ask "is this feature on?", "how many
sessions left?", and "what stage is the trial in?" without re-implementing
the read against useBillingStore.

- useFeature(flagKey): boolean — reads enabledFeatures from store
- useFeatureLimit(field): { used, limit, percentage, isAtLimit, isLoading }
  with non-blocking 60s module-level cache and graceful 404 degradation
- useTrialBanner(): derives stage from subscription status + trial countdown,
  returns null on initial load to prevent flicker
- usageApi.getCount(field) — calls /api/v1/usage/{field}; backend endpoint
  is not yet implemented (planned), so the hook degrades to used=0

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

frontend/src/api/index.ts

@@ -10,6 +10,7 @@ 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 usageApi } from './usage'
 export { default as adminApi } from './admin'
 export { treeMarkdownApi } from './treeMarkdown'
 export { default as analyticsApi } from './analytics'