feat(l1): register /l1/* routes + L1RouteGuard + page stubs

L1RouteGuard wraps the new routes and redirects users without
canUseL1Surface back to /. Page components are stubs in this task
(real UI in T21-T24): L1Dashboard, L1WalkPage, L1DraftsPage,
L1TicketsPage.

Routes: /l1, /l1/walk/:sessionId, /l1/drafts, /l1/tickets — all gated.

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

frontend/src/components/layout/L1RouteGuard.tsx

@@ -0,0 +1,10 @@
+import { Navigate } from 'react-router-dom'
+import { usePermissions } from '@/hooks/usePermissions'
+
+export function L1RouteGuard({ children }: { children: React.ReactNode }) {
+  const { canUseL1Surface } = usePermissions()
+  if (!canUseL1Surface) {
+    return <Navigate to="/" replace />
+  }
+  return <>{children}</>
+}