Add admin dashboard shell with sidebar navigation and page scaffolds.

Replace the default Next.js landing page with a client-side AppShell layout, placeholder pages for each admin section, and UI utility dependencies.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/components/layout/AppShell.tsx

@@ -0,0 +1,52 @@
+"use client";
+
+import { useState } from "react";
+import { Sidebar } from "./Sidebar";
+import { Topbar } from "./Topbar";
+
+import { HomePage } from "@/components/pages/HomePage";
+import { AssistantPage } from "@/components/pages/AssistantPage";
+import { ComponentsPage } from "@/components/pages/ComponentsPage";
+import { HistoryPage } from "@/components/pages/HistoryPage";
+import { TestPage } from "@/components/pages/TestPage";
+import { WorkflowPage } from "@/components/pages/WorkflowPage";
+import { ProfilePage } from "@/components/pages/ProfilePage";
+
+export type NavKey =
+  | "home"
+  | "assistants"
+  | "components"
+  | "history"
+  | "test"
+  | "workflow"
+  | "profile";
+
+export function AppShell() {
+  const [active, setActive] = useState<NavKey>("home");
+  const [collapsed, setCollapsed] = useState(false);
+
+  return (
+    <div className="flex h-screen overflow-hidden bg-[#080b13] text-[#e9eef7]">
+      <Sidebar
+        active={active}
+        collapsed={collapsed}
+        onNavigate={setActive}
+        onToggle={() => setCollapsed((v) => !v)}
+      />
+
+      <main className="flex min-w-0 flex-1 flex-col overflow-hidden">
+        <Topbar />
+
+        <div className="flex-1 overflow-y-auto px-8 py-7">
+          {active === "home" && <HomePage />}
+          {active === "assistants" && <AssistantPage />}
+          {active === "components" && <ComponentsPage />}
+          {active === "history" && <HistoryPage />}
+          {active === "test" && <TestPage />}
+          {active === "workflow" && <WorkflowPage />}
+          {active === "profile" && <ProfilePage />}
+        </div>
+      </main>
+    </div>
+  );
+}