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/components/index/About.vue

@@ -1,17 +0,0 @@
-<template>
-  <div>
-    <!-- 关于我们 -->
-    <section id="about" class="max-w-6xl mx-auto py-16 px-4">
-      <h3 class="text-2xl font-bold text-gray-900 mb-4">关于校友会</h3>
-      <p class="text-gray-700 leading-relaxed">
-        永平中学校友会成立的宗旨是连接全球校友，传承母校精神，支持在校学生成长。我们定期举办活动，推动交流与合作，并积极回馈母校。
-      </p>
-    </section>
-  </div>
-</template>
-
-<script lang="ts" setup>
-
-</script>
-
-<style></style>