feat(billing): add FeatureGate, UpgradePrompt, EmailVerificationGate components

Three drop-in gating components for the self-serve signup flow.

- FeatureGate reads useFeature(flag) and renders children when enabled,
  else a fallback (default UpgradePrompt). UX-only — security boundary
  remains require_feature on the backend.
- UpgradePrompt resolves a feature key to display name + required plan
  via an inline catalog and links to /account/billing/select-plan.
- EmailVerificationGate gates protected content behind a 6-day grace
  period; renders a minimal EmailVerificationWall (resend + sign out)
  on Day 7+ unverified. Wall design will be refined in Task 37.

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

frontend/src/components/common/FeatureGate.tsx

@@ -0,0 +1,42 @@
+import type { ReactNode } from 'react'
+import { useFeature } from '@/hooks/useFeature'
+import { UpgradePrompt } from './UpgradePrompt'
+
+interface FeatureGateProps {
+  /** Feature flag key (e.g. `psa_integration`). Must match a backend `feature_flags.flag_key`. */
+  feature: string
+  /**
+   * Rendered when the feature is enabled for the current account.
+   */
+  children: ReactNode
+  /**
+   * Rendered when the feature is disabled. Defaults to `<UpgradePrompt feature={feature} />`.
+   * Pass `null` to render nothing.
+   */
+  fallback?: ReactNode
+}
+
+/**
+ * Conditionally renders `children` based on whether `feature` is enabled
+ * for the current account.
+ *
+ * This is a UX affordance — the security boundary is the backend
+ * `require_feature` dependency. Never trust this gate for authorization.
+ */
+export function FeatureGate({ feature, children, fallback }: FeatureGateProps) {
+  const enabled = useFeature(feature)
+
+  if (enabled) {
+    return <>{children}</>
+  }
+
+  // Use explicit fallback when provided, otherwise render the standard prompt.
+  // `null` is a valid fallback (renders nothing).
+  if (fallback !== undefined) {
+    return <>{fallback}</>
+  }
+
+  return <UpgradePrompt feature={feature} />
+}
+
+export default FeatureGate