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/types/billing.ts

@@ -0,0 +1,51 @@
+/**
+ * Billing state types for the unified `/billing/state` endpoint.
+ *
+ * The backend returns snake_case keys (`plan_billing`, `enabled_features`);
+ * the API client (`frontend/src/api/billing.ts`) transforms the payload to
+ * camelCase before it reaches the rest of the frontend.
+ */
+
+export type SubscriptionStatus =
+  | 'trialing'
+  | 'active'
+  | 'past_due'
+  | 'canceled'
+  | 'incomplete'
+  | 'complimentary'
+
+export interface SubscriptionState {
+  status: SubscriptionStatus
+  plan: string
+  /** ISO 8601 string or null */
+  current_period_start: string | null
+  /** ISO 8601 string or null */
+  current_period_end: string | null
+  cancel_at_period_end: boolean
+  seat_limit: number | null
+  has_pro_entitlement: boolean
+  is_paid: boolean
+}
+
+export interface PlanBillingState {
+  display_name: string
+  description: string | null
+  monthly_price_cents: number | null
+  annual_price_cents: number | null
+}
+
+/** Camel-cased billing-state payload, post-transform. */
+export interface BillingStatePayload {
+  subscription: SubscriptionState | null
+  planBilling: PlanBillingState | null
+  planLimits: Record<string, unknown>
+  enabledFeatures: Record<string, boolean>
+}
+
+/** Raw snake_case payload returned by the backend. */
+export interface BillingStateApiResponse {
+  subscription: SubscriptionState | null
+  plan_billing: PlanBillingState | null
+  plan_limits: Record<string, unknown>
+  enabled_features: Record<string, boolean>
+}

frontend/src/types/index.ts

@@ -93,6 +93,14 @@ export type {
   KBQuotaResponse,
 } from './kbAccelerator'
 
+export type {
+  SubscriptionStatus,
+  SubscriptionState as BillingSubscriptionState,
+  PlanBillingState,
+  BillingStatePayload,
+  BillingStateApiResponse,
+} from './billing'
+
 export * from './scripts'
 export * from './script-builder'
 export * from './integrations'