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.vue

@@ -0,0 +1,19 @@
+<template>
+  <UContainer>
+    <UDashboardToolbar>
+      <UNavigationMenu :items="subPages" />
+    </UDashboardToolbar>
+    <NuxtPage />
+  </UContainer>
+</template>
+
+<script lang="ts" setup>
+import type { NavigationMenuItem } from "@nuxt/ui";
+
+const subPages = ref<NavigationMenuItem[]>([
+  { label: "庆祝四十载", to: "/40th-anniversary", exact: true },
+  { label: "策划案", to: "/40th-anniversary/proposal" },
+]);
+</script>
+
+<style></style>