feat(app): restructure to multi-page layout and add content pages

Converts the website from a single-page design with anchor links to a full multi-page application. This change improves site organization, navigation, and scalability.

- Adds new top-level pages: `/news`, `/events`, and `/about`.
- Introduces a new section for the 40th Anniversary at `/40th-anniversary`, including a proposal sub-page.
- Updates the default layout with a new navigation menu, a promotional banner, social links, and page transitions.

app/pages/index.vue

@@ -1,5 +1,5 @@
 <template>
-  <div>
+  <UPage>
     <!-- Hero Banner -->
     <UPageHero
       class="bg-cover bg-center"
@@ -28,9 +28,7 @@
       description="您的捐赠将用于奖学金、校园建设及校友活动发展。感谢您对母校的支持！"
       :links="donationLinks"
     />
-
-    <IndexAbout />
-  </div>
+  </UPage>
 </template>
 
 <script lang="ts" setup>