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/components/layout/AppLayout.tsx

@@ -4,6 +4,7 @@ import { Menu, X, LayoutGrid, Clock, AlertTriangle, GitBranch, Wand2, BarChart3,
 import { useAuthStore } from '@/store/authStore'
 import { usePermissions } from '@/hooks/usePermissions'
 import { useUserPreferencesStore } from '@/store/userPreferencesStore'
+import { useBillingPoll } from '@/hooks/useBillingPoll'
 import { BrandLogo } from '@/components/common/BrandLogo'
 import { TopBar } from './TopBar'
 import { Sidebar } from './Sidebar'
@@ -13,6 +14,9 @@ import { FeedbackWidget } from '@/components/common/FeedbackWidget'
 import { cn } from '@/lib/utils'
 
 export function AppLayout() {
+  // Poll /billing/state every 60s while authenticated. Hook no-ops when logged out.
+  useBillingPoll()
+
   const location = useLocation()
   const navigate = useNavigate()
   const { user, logout } = useAuthStore()