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>

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: [