feat: add hidden /portal page for SinoCode iframe embedding

Full-screen landing page with glassmorphism cards linking to monitoring
(8018), docs (8017), and analytics (8019). Sidebar is conditionally
hidden on the portal route. Supports dark/light themes and i18n.

app/portal/layout.tsx

@@ -0,0 +1,14 @@
+import type { Metadata } from "next";
+
+export const metadata: Metadata = {
+  title: "SinoCode — 智能 API 网关平台",
+  description: "SinoCode intelligent API gateway platform portal",
+};
+
+export default function PortalLayout({
+  children,
+}: {
+  children: React.ReactNode;
+}) {
+  return <>{children}</>;
+}