feat: add "Add Referral" button to referrals tab

- New search UI in referrals list section to find and bind users
  as this user's referrals (calls assign-referrer on target user)
- Debounced search with cancelled flag and loading state
- Click-outside to close dropdown
- i18n: addReferral, referralAdded keys in all 4 locales

src/locales/zh.json

@@ -2735,6 +2735,8 @@
           "noReferrals": "暂无推荐",
           "removeReferral": "移除推荐",
           "referralRemoved": "推荐已移除",
+          "addReferral": "添加推荐",
+          "referralAdded": "推荐已添加",
           "searchPlaceholder": "按名称、邮箱或 Telegram ID 搜索...",
           "alreadyReferred": "已有推荐人",
           "noUsersFound": "未找到用户"