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/40th-anniversary/index.vue

@@ -0,0 +1,13 @@
+<template>
+  <div>
+    40 周年庆纪念页
+  </div>
+</template>
+
+<script lang="ts" setup>
+
+</script>
+
+<style>
+
+</style>