feat(01-code-splitting): add feature gating with ALWRITY_ENABLED_FEATURES

- Create FeatureRoute.tsx wrapper component for route-level feature gating
- Add FEATURE_KEYS constant map to demoMode.ts for type-safe feature references
- Wrap 47 feature-specific routes with <FeatureRoute> in App.tsx
- Core routes (dashboard, billing, pricing, auth callbacks) remain ungated
- Disabled features redirect to /dashboard and never load their lazy chunks
- Main bundle: +259 bytes (FeatureRoute is a lightweight component)

Closes Phase 1 Plan 01-02

frontend/src/components/shared/FeatureRoute.tsx

@@ -0,0 +1,34 @@
+import React from 'react';
+import { Navigate } from 'react-router-dom';
+import { isFeatureEnabled } from '../../utils/demoMode';
+
+interface FeatureRouteProps {
+  feature: string;
+  children: React.ReactNode;
+  /** Where to redirect if feature is disabled (default: /dashboard) */
+  redirectTo?: string;
+}
+
+/**
+ * Route guard that checks if a feature is enabled via ALWRITY_ENABLED_FEATURES.
+ * If disabled, redirects to the fallback route and the lazy chunk never loads.
+ *
+ * Usage:
+ *   <Route path="/blog-writer" element={
+ *     <ProtectedRoute>
+ *       <FeatureRoute feature="blog_writer"><BlogWriter /></FeatureRoute>
+ *     </ProtectedRoute>
+ *   } />
+ */
+const FeatureRoute: React.FC<FeatureRouteProps> = ({ 
+  feature, 
+  children, 
+  redirectTo = '/dashboard' 
+}) => {
+  if (!isFeatureEnabled(feature)) {
+    return <Navigate to={redirectTo} replace />;
+  }
+  return <>{children}</>;
+};
+
+export default FeatureRoute;