feat(user-center): redesign 推广中心 panel + extract promotion.css

Give the standalone promotion tab its own stylesheet and a task-priority
layout (成果概览 → 分享操作 → 奖励说明 → 邀请记录).

- assets/css/routes/user-center/promotion.css: new module owning all
  uc-promotion-* rules; flattens #panel-promotion chrome, adds focus-visible
  on the copy button + link input, and a ≤640px single-column layout with a
  full-width copy button (no horizontal overflow)
- user-center.css: import the new promotion.css
- user-center/revenue.css: drop the now-migrated uc-promotion-* rules (keep
  only the revenue-layout mobile rule)
- profile-invite.php: refresh doc header to describe the section priority
  order; copy button stays a focusable <button>, credits.js keeps text-based
  success/failure feedback via [data-invite-copy-feedback]

inc/helpers/user-center-renderers/profile-invite.php

@@ -1,6 +1,9 @@
 <?php
 /**
- * User center revenue invitation section.
+ * User center promotion (invite) card.
+ *
+ * Rendered inside the standalone 推广中心 tab (profile-promotion.php).
+ * Section order follows the task priority: 成果概览 → 分享操作 → 奖励说明 → 邀请记录.
  *
  * @package AKSRT
  */