feat: token auto-refresh, studio→render wiring, admin panel (nodes + render queue)

Token auto-refresh (middleware):
- Proactively refresh fr_access when < 120s remain — no more silent 15-min kick
- Inlines /v1/auth/refresh call in middleware, stamps new cookies on response
- /admin/* protected: is_admin JWT claim required, else redirect /dashboard
- apiFetch() (src/lib/api/fetch.ts): client-side 401 → auto-refresh → retry;
  de-duplicates concurrent refresh calls; redirects to /auth on failure

Studio → Render V2 wiring:
- scenes[] no longer sent to POST /api/render (V2 render-svc fetches project
  from Studio service via saved_project_id directly)
- renderRequestSchema.scenes is now optional
- RenderModal uses apiFetch for auto-refresh on 401 during polling

Admin panel (/admin/*):
- Admin layout: server-side is_admin guard + top nav (Nodes, Render Queue)
- /admin/nodes: lists all nodes from GET /v1/nodes with status badges,
  heartbeat age, slot usage, tags; Drain (PATCH status=Draining) + Release actions
- /admin/renders: render job table with step filter tabs; progress bars,
  error messages, Retry + Cancel per-row actions; polls GET /v1/renders
- API proxy routes: /api/admin/nodes/:id/drain|release,
  /api/admin/renders/:id/retry|cancel — all validate is_admin in JWT before proxying

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

src/app/[locale]/admin/layout.tsx

@@ -0,0 +1,44 @@
+import { redirect } from "next/navigation";
+
+import { getCurrentUser } from "@/lib/auth/session";
+
+export const dynamic = "force-dynamic";
+
+export default async function AdminLayout({
+  children,
+}: {
+  children: React.ReactNode;
+}) {
+  const user = await getCurrentUser();
+  if (!user || !user.is_admin) {
+    redirect("/dashboard");
+  }
+  return (
+    <div className="min-h-screen bg-[#0c0e1a] text-gray-200">
+      <nav className="border-b border-[#1e2235] bg-[#0f1120] px-6 py-3">
+        <div className="mx-auto flex max-w-7xl items-center gap-6">
+          <span className="text-sm font-semibold text-white">FlatRender Admin</span>
+          <a
+            href="/admin/nodes"
+            className="text-sm text-gray-400 hover:text-white transition-colors"
+          >
+            Nodes
+          </a>
+          <a
+            href="/admin/renders"
+            className="text-sm text-gray-400 hover:text-white transition-colors"
+          >
+            Render Queue
+          </a>
+          <a
+            href="/dashboard"
+            className="ml-auto text-xs text-gray-500 hover:text-gray-300 transition-colors"
+          >
+            ← Back to Dashboard
+          </a>
+        </div>
+      </nav>
+      <main className="mx-auto max-w-7xl px-6 py-8">{children}</main>
+    </div>
+  );
+}