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/members/index.vue

@@ -77,12 +77,12 @@ const columns: TableColumn<Member>[] = [
           // 初中毕业
           // 如果 row.original.graduateYear 不能转换成数字，就写成初中毕业
           // 否则计算届别
-          return isNaN(Number(row.original.graduateYear))
+          return isNaN(Number(row.original.graduateYear)) || row.original.graduateYear.trim() == ""
             ? "初中毕业"
             : `初中第 ${Number(row.original.graduateYear) - 1958} 届`;
         case "s":
           // 高中毕业
-          return isNaN(Number(row.original.graduateYear))
+          return isNaN(Number(row.original.graduateYear)) || row.original.graduateYear.trim() == ""
             ? "高中毕业"
             : `高中第 ${Number(row.original.graduateYear) - 1965} 届`;
         case "dj1":