feat(06-01): replace top NavBar with AppShell flex-row sidebar layout

- AdminLayout now uses flex-row: AdminSidebar (left) + main (right)
- Removed NavBar import and max-w-5xl centered constraint
- Main content area takes remaining width with px-8 py-8 padding

src/app/admin/layout.tsx

@@ -1,4 +1,4 @@
-import { NavBar } from "@/components/admin/NavBar";
+import { AdminSidebar } from "@/components/admin/AdminSidebar";
 import { getServerSession } from "next-auth";
 import { authOptions } from "@/lib/auth";
 
@@ -9,9 +9,11 @@ export default async function AdminLayout({
 }) {
   const session = await getServerSession(authOptions);
   return (
-    <div className="min-h-screen bg-gray-50">
+    <div className="flex min-h-screen bg-gray-50">
-      {session && <NavBar />}
+      {session && <AdminSidebar />}
-      <main className="max-w-5xl mx-auto px-6 py-8">{children}</main>
+      <main className="flex-1 px-8 py-8 overflow-y-auto">
+        {children}
+      </main>
     </div>
   );
 }