feat(ui): external promotion dashboard (campaigns.html)

Add campaigns.html with 6 tabs (Overview/Products/Campaigns/Rankings/Audits/Tasks) using Vue 3 + Element Plus CDN. Add nav entry to uni-nav.js.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

platform/frontend/uni-nav.js

@@ -132,6 +132,7 @@
       { key: 'factory', label: '内容工厂', page: 'factory.html' },
       { key: 'insights', label: '数据洞察', page: 'insights.html' },
       { key: 'assets', label: '资产库', page: 'assets.html' },
+      { key: 'campaigns', label: '外推营销', page: 'campaigns.html' },
     ];
     if (isAdmin) {
       items.push({ key: 'admin', label: '系统管理', page: 'admin.html', admin: true });