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/lib/cms/content-types.ts

@@ -453,6 +453,12 @@ const standaloneProductFields: FieldDefinition[] = [
     label: '产品图片',
     required: true,
   },
+  {
+    name: 'externalUrl',
+    type: 'text',
+    label: '外部跳转链接',
+    description: '若设置，访问该产品详情页时将跳转到此 URL（如独立子域名产品）',
+  },
   {
     name: 'category',
     type: 'text',