feat: add useFeatureFlag hook for feature gating

Selector-based hook reads from authStore.featureFlags.
Returns false for unknown keys (fail closed).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/hooks/useFeatureFlag.ts

@@ -0,0 +1,9 @@
+import { useAuthStore } from '@/store/authStore'
+
+/**
+ * Check if a feature flag is enabled for the current user.
+ * Returns false for unknown keys (fail closed).
+ */
+export function useFeatureFlag(key: string): boolean {
+  return useAuthStore((s) => s.featureFlags[key] ?? false)
+}