fix(products): add externalUrl support for standalone products and fix footer alignment

- Add externalUrl field to standalone product content type
- Filter out externalUrl products from generateStaticParams
- Fix email icon vertical alignment in footer

src/app/(marketing)/products/standalone/[id]/page.tsx

@@ -8,7 +8,10 @@ export const revalidate = 3600;
 
 export async function generateStaticParams() {
   const items = await getPublishedItems('standalone-product');
-  return items.map((item) => ({ id: item.slug! }));
+  // 排除有 externalUrl 的产品（它们会跳转到外部域名，无需生成静态页面）
+  return items
+    .filter((item) => !item.data.externalUrl)
+    .map((item) => ({ id: item.slug! }));
 }
 
 export async function generateMetadata({ params }: { params: Promise<{ id: string }> }): Promise<Metadata> {