feat(seo): implement SEO enhancements and social media embeds

This commit introduces significant SEO improvements and enhances content embedding capabilities.

- Integrates `@nuxtjs/seo`, `@nuxtjs/sitemap`, and `@nuxtjs/robots` to boost search engine visibility.
- Configures global meta tags, including Open Graph and Twitter Cards, for richer social media sharing.
- Adds support for embedding YouTube and Facebook content directly within markdown pages.
- Introduces a new `TikTokEmbed` component for future use.

app/components/TikTokEmbed.vue

@@ -0,0 +1,26 @@
+<script setup lang="ts">
+import { onMounted } from "vue";
+
+const props = defineProps<{
+  url: string
+}>()
+
+onMounted(() => {
+  // 如果 TikTok embed 脚本未加载，动态注入
+  if (!document.querySelector('script[src="https://www.tiktok.com/embed.js"]')) {
+    const s = document.createElement("script");
+    s.src = "https://www.tiktok.com/embed.js";
+    s.async = true;
+    document.body.appendChild(s);
+  } else {
+    // 如果脚本已存在，调用它刷新 embed
+    // 官方脚本会自动处理，不需要手动 init
+  }
+});
+</script>
+
+<template>
+  <blockquote class="tiktok-embed" :cite="url" data-video-id="" style="max-width: 605px; min-width: 325px;">
+    <a :href="url">View on TikTok</a>
+  </blockquote>
+</template>