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> {

src/components/layout/footer.tsx

@@ -76,8 +76,8 @@ export function Footer() {
                 <MapPin className="w-4 h-4 text-gray-400 mt-0.5 shrink-0" />
                 <span className="text-gray-300 text-sm leading-relaxed">{config.address}</span>
               </li>
-              <li className="flex items-center gap-3">
-                <Mail className="w-4 h-4 text-gray-400 shrink-0" />
+              <li className="flex items-start gap-3">
+                <Mail className="w-4 h-4 text-gray-400 mt-0.5 shrink-0" />
                 <a 
                   href={`mailto:${config.email}`}
                   className="text-gray-300 hover:text-white transition-colors duration-200 text-sm"

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',