feat(categories): implement category management with hierarchical structure and update product association

frontend/app/api/categories/route.ts

@@ -0,0 +1,12 @@
+import { NextResponse } from 'next/server';
+
+const API_BASE = process.env.NEXT_PUBLIC_API_URL_INTERNAL || 'http://recipe-api:8080';
+
+export async function GET() {
+  const res = await fetch(`${API_BASE}/api/categories/tree`, { cache: 'no-store' });
+  const text = await res.text();
+  return new NextResponse(text, {
+    status: res.status,
+    headers: { 'Content-Type': 'application/json' },
+  });
+}