refactor: 备案号改为环境变量配置

ICP 和公安备案号通过 NUXT_PUBLIC_ICP / NUXT_PUBLIC_PS_BEIAN 环境变量读取，
空值时底部不显示。

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.env.example

@@ -1,3 +1,3 @@
-# 站点配置（可选，config/site.ts 中已有默认值）
-NUXT_PUBLIC_SITE_URL=https://2x.nz
-NUXT_PUBLIC_SITE_NAME=《二叉树树》官方网站
+# 备案号（必填，否则底部不显示）
+NUXT_PUBLIC_ICP=蜀ICP备XXXXXXXXXX号
+NUXT_PUBLIC_PS_BEIAN=51015602001952

components/layout/Footer.vue

@@ -1,19 +1,25 @@
 <script setup lang="ts">
 import { siteConfig } from '~/config/site'
+
+const config = useRuntimeConfig()
+const icp = config.public.icp
+const psBeian = config.public.psBeian
 </script>
 
 <template>
   <footer class="py-6 text-center text-sm border-t" style="color: var(--color-text-secondary); border-color: var(--color-border);">
     <p>
       &copy; {{ new Date().getFullYear() }} {{ siteConfig.siteName }}
-      <template v-if="siteConfig.icp">
+      <template v-if="icp">
         &nbsp;·&nbsp;
-        <a href="https://beian.miit.gov.cn/" target="_blank" rel="noopener noreferrer" class="hover:opacity-70" style="border-bottom: 1px solid var(--color-text-secondary); padding-bottom: 1px;">{{ siteConfig.icp }}</a>
+        <a href="https://beian.miit.gov.cn/" target="_blank" rel="noopener noreferrer" class="hover:opacity-70" style="border-bottom: 1px solid var(--color-text-secondary); padding-bottom: 1px;">{{ icp }}</a>
       </template>
+      <template v-if="psBeian">
         &nbsp;·&nbsp;
-      <a href="https://beian.mps.gov.cn/#/query/webSearch?code=51015602001952" rel="noreferrer" target="_blank" class="hover:opacity-70" style="border-bottom: 1px solid var(--color-text-secondary); padding-bottom: 1px;">
-        <img src="@/assets/备案图标.png" alt="" class="w-3 h-3 inline" />川公网安备51015602001952号
+        <a :href="`https://beian.mps.gov.cn/#/query/webSearch?code=${psBeian}`" rel="noreferrer" target="_blank" class="hover:opacity-70" style="border-bottom: 1px solid var(--color-text-secondary); padding-bottom: 1px;">
+          <img src="@/assets/备案图标.png" alt="" class="w-3 h-3 inline" />川公网安备{{ psBeian }}号
         </a>
+      </template>
     </p>
   </footer>
 </template>

nuxt.config.ts

@@ -6,6 +6,13 @@ export default defineNuxtConfig({
   compatibilityDate: '2025-01-01',
   devtools: { enabled: true },
 
+  runtimeConfig: {
+    public: {
+      icp: '',
+      psBeian: '',
+    },
+  },
+
   css: ['~/assets/css/main.css'],
 
   modules: [