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/pages/l1/L1WalkPage.tsx

@@ -0,0 +1,13 @@
+import { PageMeta } from '@/components/common/PageMeta'
+
+export default function L1WalkPage() {
+  return (
+    <div className="overflow-y-auto h-full">
+      <PageMeta title="L1 Walk" />
+      <div className="max-w-4xl mx-auto px-6 pt-12 pb-12">
+        <h1 className="font-heading text-2xl font-bold">L1 Walk</h1>
+        <p className="text-muted-foreground mt-2">Loading…</p>
+      </div>
+    </div>
+  )
+}