fix: React Router basename - use ?? instead of || for empty string support

When SPA is frontpage, basePath is empty string. JavaScript || treats '' as falsy
and falls back to /store. Changed to ?? (nullish coalescing) so empty string works.

customer-spa/src/App.tsx

@@ -104,7 +104,7 @@ const getRouterConfig = () => {
   const config = (window as any).woonoowCustomer;
   return {
     useBrowserRouter: config?.useBrowserRouter ?? true,
-    basePath: config?.basePath || '/store',
+    basePath: config?.basePath ?? '/store',
   };
 };