feat(layout): add ProblemsetLayout component with Banner and main content wrapper

src/app/(app)/problemset/layout.tsx

@@ -0,0 +1,16 @@
+import { Banner } from "@/components/banner";
+
+interface ProblemsetLayoutProps {
+  children: React.ReactNode;
+}
+
+export default function ProblemsetLayout({ children }: ProblemsetLayoutProps) {
+  return (
+    <div className="flex flex-col">
+      <Banner />
+      <main className="container mx-auto p-4">
+        {children}
+      </main>
+    </div>
+  );
+}