feat(seo): improve SEO and add analytics tracking

This commit introduces several enhancements for SEO and analytics.

- Updates the canonical site URL from `yphsalumni.com` to `yphsalumni.org`.
- Enriches the 'middle-highschool-tuition-class' page with detailed Open Graph (og) meta tags for better social sharing.
- Implements Umami event tracking for the 'Join Us' button on the homepage.

app/pages/about/middle-highschool-tuition-class.vue

@@ -79,8 +79,16 @@
 
 <script lang="ts" setup>
 useSeoMeta({
-  title: "致中补班的一封信",
+  title: "致：1956年永中中补班 — 永平中学校友会",
   ogTitle: "致中补班的一封信",
+  ogDescription:
+    "致亲爱的1956年‘诚’、‘毅’两班老校友：虽迟六十余年，愿这条讯息穿越时间，召集大家重聚，共叙曾经校园情怀与夕阳红岁月。",
+  ogUrl: "https://yphsalumni.org/about/middle-highschool-tuition-class/",
+  ogType: "article",
+  ogImage: "/about/中补班乙.jpg",
+  ogImageAlt: "永平学校中辅乙同学合影",
+  articlePublishedTime: "2022-04-15T00:00:00+08:00",
+  author: "永平中学校友会",
 });
 
 const imagePopup = ref<boolean>(false);