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

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+<template>
+  <div>
+    40 周年庆纪念页
+  </div>
+</template>
+
+<script lang="ts" setup>
+
+</script>
+
+<style>
+
+</style>

app/pages/40th-anniversary/proposal.vue

@@ -0,0 +1,58 @@
+<template>
+  <UPage>
+    <UPageBody>
+      <UContainer>
+        <UPage>
+          <template #default>
+            <UStepper :items="proposalSteps" size="xs" disabled/>
+            <UPageHeader
+              title="永平中学校友会 40 周年纪念册筹划"
+              description="四十载薪火相传，情系永平，共创未来"
+            />
+
+            <UPageSection title="纪念册定位">
+              <h3>目标受众</h3>
+              <ul>
+                <li>各届校友（老中青三代）</li>
+                <li>教职员工与校董会</li>
+                <li>在校学生与家长</li>
+                <li>地区社会贤达 / 赞助商 / 友校代表</li>
+              </ul>
+              <h3>风格方向</h3>
+              <p>庄重 × 情感 × 历史厚度 × 现代视觉感</p>
+              <p>
+                → 类似大学纪念刊风格，不是单纯的活动册，而是一部
+                *时代见证作品*。
+              </p>
+            </UPageSection>
+
+            <UPageSection
+              title="总体结构规划"
+              description="建议页数：120～160 页"
+            >
+            </UPageSection>
+          </template>
+        </UPage>
+      </UContainer>
+    </UPageBody>
+  </UPage>
+</template>
+
+<script lang="ts" setup>
+import type { StepperItem } from "@nuxt/ui";
+
+useSeoMeta({
+  title: "40 周年纪念册筹划",
+});
+
+const proposalSteps = ref<StepperItem[]>([
+  { title: "筹划期", description: "成立编辑组，确定风格、预算、印刷规格" },
+  { title: "资料收集期", description: "访谈、征文、收照片、整理档案" },
+  { title: "撰写与设计期", description: "文稿成稿、图片修复、初稿排版" },
+  { title: "审校与赞助期", description: "校对、内容确认、广告页洽谈" },
+  { title: "印刷准备期", description: "定稿送印、样书确认" },
+  { title: "发布期", description: "校庆活动同步发行、媒体推广、线上版本上线" },
+]);
+</script>
+
+<style></style>