fixed platform selector and part selector

app/(builder)/builder/page.tsx

@@ -87,7 +87,16 @@ export default function GunbuilderPage() {
   const [parts, setParts] = useState<Part[]>([]);
   const [loading, setLoading] = useState(true);
   const [error, setError] = useState<string | null>(null);
-  const [platform, setPlatform] = useState<(typeof PLATFORMS)[number]>("AR-15");
+  const isValidPlatform = (
+    value: string | null
+  ): value is (typeof PLATFORMS)[number] =>
+    !!value && (PLATFORMS as readonly string[]).includes(value);
+
+  const [platform, setPlatform] = useState<(typeof PLATFORMS)[number]>(() => {
+    if (typeof window === "undefined") return "AR-15";
+    const initial = new URLSearchParams(window.location.search).get("platform");
+    return isValidPlatform(initial) ? initial : "AR-15";
+  });
   const [build, setBuild] = useState<BuildState>(() => {
     if (typeof window !== "undefined") {
       const stored = localStorage.getItem(STORAGE_KEY);
@@ -189,11 +198,26 @@ export default function GunbuilderPage() {
           return updated;
         });
 
-        router.replace("/builder", { scroll: false });
+        const qp = searchParams.get("platform");
+        const nextPlatform = isValidPlatform(qp) ? qp : platform;
+
+        router.replace(
+          `/builder?platform=${encodeURIComponent(nextPlatform)}`,
+          {
+            scroll: false,
+          }
+        );
       }
     }
   }, [searchParams, router]);
 
+  useEffect(() => {
+    const qp = searchParams.get("platform");
+    if (isValidPlatform(qp) && qp !== platform) {
+      setPlatform(qp);
+    }
+  }, [searchParams, platform]);
+
   // Persist build state to localStorage whenever it changes
   useEffect(() => {
     if (typeof window !== "undefined") {
@@ -695,10 +719,24 @@ export default function GunbuilderPage() {
                                       <Link
                                         href={{
                                           pathname: `/builder/${category.id}`,
-                                          query: { platform },
+                                          query: platform ? { platform } : {},
                                         }}
+                                        className="inline-flex items-center gap-1.5 whitespace-nowrap rounded-md border border-zinc-700 bg-zinc-900/70 px-3 py-1.5 text-xs font-medium text-zinc-300 hover:bg-zinc-800 hover:border-zinc-600 transition-colors"
                                       >
-                                        Choose a Part
+                                        <svg
+                                          className="w-3.5 h-3.5"
+                                          fill="none"
+                                          stroke="currentColor"
+                                          viewBox="0 0 24 24"
+                                        >
+                                          <path
+                                            strokeLinecap="round"
+                                            strokeLinejoin="round"
+                                            strokeWidth={2}
+                                            d="M12 4v16m8-8H4"
+                                          />
+                                        </svg>
+                                        <span>Choose a Part</span>
                                       </Link>
                                     ) : (
                                       <span className="text-[0.7rem] text-zinc-600">