refactor(admin): retire settings migration and compat menu demotion

Stop auto-running AKSRT_Settings_Migration, drop multi-option settings
merge and Zibll favicon aliases, remove the migration tools entry from
menus/schema, flatten admin sections so pages are not demoted into a
compat bag, and retire the migration-status payload for leftover SPA
routes after a full admin rebuild.

assets/admin/src/pages/tools/ToolsBackup.vue

@@ -7,7 +7,6 @@
       <template #actions>
         <n-space>
           <n-button quaternary @click="go('/tools/import-export')">导入导出</n-button>
-          <n-button quaternary @click="go('/tools/migration')">迁移状态</n-button>
         </n-space>
       </template>
     </PageHeader>

assets/admin/src/pages/tools/ToolsImportExport.vue

@@ -8,7 +8,6 @@
         <n-space>
           <n-button quaternary @click="go('/tools/backup')">数据备份</n-button>
           <n-button quaternary @click="go('/tools/health')">健康检查</n-button>
-          <n-button quaternary @click="go('/tools/migration')">迁移状态</n-button>
         </n-space>
       </template>
     </PageHeader>

assets/admin/src/pages/tools/ToolsUpdate.vue

@@ -87,7 +87,6 @@ const guides = [
   { key: 'docs-mode', title: '文档模式', desc: '配置文档分类与专题模板', url: '#/extensions/docs-mode' },
   { key: 'oauth', title: '第三方登录', desc: '检查 GitHub / Gitee / QQ / Google 登录', url: '#/users/oauth' },
   { key: 'backup', title: '设置快照', desc: '创建与恢复站点配置快照', url: '#/tools/backup' },
-  { key: 'migration', title: '迁移状态', desc: '确认 schema 与设置迁移版本', url: '#/tools/migration' },
   { key: 'health', title: '健康检查', desc: '查看 PHP / WP / 数据表状态', url: '#/tools/health' },
   { key: 'meilisearch', title: 'Meilisearch', desc: '配置搜索索引与同步策略', url: '#/global/search' },
   { key: 'i18n', title: '多语言翻译', desc: '检查语言切换与术语配置', url: '#/global/i18n' },