refactor(ui): adopt Nuxt UI prose styles and apply various fixes

Replaces custom `markdown.css` with the default prose styling from Nuxt UI to simplify the styling architecture. This commit also includes several other fixes and improvements:

- fix(members): Correctly
handle empty `graduateYear` strings in the member list.
- fix(layout): Wrap `NuxtPage` in a div to resolve page transition issues.
- feat(seo): Add a meta title to the homepage.
- docs(content): Correct
the title and add an image to an event page.

app/pages/index.vue

@@ -36,6 +36,10 @@
 <script lang="ts" setup>
 import type { BlogPostProps } from "@nuxt/ui";
 
+useSeoMeta({
+  title: "首页",
+});
+
 const heroCta = ref([
   {
     label: "立即加入我们",