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.

nuxt.config.ts

@@ -68,7 +68,7 @@ export default defineNuxtConfig({
     },
   },
   site: {
-    url: "https://yphsalumni.com",
+    url: "https://yphsalumni.org",
     name: "永中校友会 YPHS Alumni",
   },
 });