fixed part role mapping to categories. set a const for categories

app/gunbuilder/[categoryId]/page.tsx

@@ -4,6 +4,7 @@ import { useEffect, useMemo, useState } from "react";
 import Link from "next/link";
 import { useParams, useRouter } from "next/navigation";
 import { CATEGORIES } from "@/data/gunbuilderParts";
+import { CATEGORY_TO_PART_ROLES } from "@/data/partRoleMappings";
 import type { CategoryId, Part } from "@/types/gunbuilder";
 
 type ViewMode = "card" | "list";
@@ -22,18 +23,6 @@ type GunbuilderProductFromApi = {
 const API_BASE_URL =
   process.env.NEXT_PUBLIC_API_BASE_URL ?? "http://localhost:8080";
 
-const CATEGORY_TO_PART_ROLES: Record<CategoryId, string[]> = {
-  upper: ["upper-receiver"],
-  barrel: ["barrel"],
-  handguard: ["handguard"],
-  chargingHandle: ["charging-handle"],
-  buffer: ["buffer-kit"],
-  lowerParts: ["lower-parts-kit"],
-  sights: ["sight"],
-  lower: ["lower-receiver"],
-  optic: ["optic"],
-  stock: ["stock"],
-};
 
 // sort options
 type SortOption = "relevance" | "price-asc" | "price-desc" | "brand-asc";
@@ -80,10 +69,11 @@ export default function CategoryPage() {
 
     async function fetchCategoryParts() {
       try {
-        setLoading(true);
+         setLoading(true);
         setError(null);
 
-        const roles = CATEGORY_TO_PART_ROLES[categoryId] ?? [];
+        // FIX: determine which backend partRoles map to this category
+    const roles = CATEGORY_TO_PART_ROLES[categoryId] ?? [];
 
         const search = new URLSearchParams();
         search.set("platform", "AR-15");