feat(seo): 结构化数据/每页元数据/列表页metadata/hreflang修正/OG分享图

- 详情页注入 Article+BreadcrumbList JSON-LD，工具页 SoftwareApplication
- 课程/技能/文章/练习 动态 generateMetadata(canonical/OG)
- 新增 skills/contents/practices/prompts/models 列表页 metadata
- 修正 hreflang(删除不存在的/en)，用 next/og 生成 OG 分享图

frontend/src/app/tools/[slug]/page.tsx

@@ -1,6 +1,7 @@
 import { Metadata } from 'next';
 import { notFound } from 'next/navigation';
 import Link from 'next/link';
+import { SITE_URL } from '@/lib/config';
 import { Badge } from '@/components/ui/badge';
 import { Button } from '@/components/ui/button';
 import { Card } from '@/components/ui/card';
@@ -73,9 +74,17 @@ export default async function ToolDetailPage({ params }: { params: { slug: strin
     '@context': 'https://schema.org',
     '@type': 'SoftwareApplication',
     name: tool.name,
-    description: tool.description,
+    description: tool.description || '',
     url: tool.url,
-    applicationCategory: 'AI工具',
+    applicationCategory: 'UtilitiesApplication',
+    operatingSystem: 'Web',
+    offers: { '@type': 'Offer', price: '0', priceCurrency: 'CNY' },
+    publisher: {
+      '@type': 'Organization',
+      name: '宇之然 AI',
+      url: SITE_URL,
+      logo: { '@type': 'ImageObject', url: `${SITE_URL}/favicon.png` },
+    },
   };
 
   // 解析描述中的结构化字段