Implement recipe retrieval methods and enhance inventory item types for better data handling

frontend/app/api/recipe-preview-proxy/route.ts

@@ -5,6 +5,13 @@ const API_BASE = process.env.NEXT_PUBLIC_API_URL_INTERNAL || 'http://recipe-api:
 export async function GET(request: NextRequest) {
   const id = request.nextUrl.searchParams.get('id');
 
+  if (!id) {
+    return NextResponse.json(
+      { error: 'Missing id parameter' },
+      { status: 400 }
+    );
+  }
+
   const res = await fetch(`${API_BASE}/api/recipes/${id}/inventory-preview`, {
     method: 'GET',
     cache: 'no-store',