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/en.json

@@ -3118,6 +3118,8 @@
           "noReferrals": "No referrals yet",
           "removeReferral": "Remove referral",
           "referralRemoved": "Referral removed successfully",
+          "addReferral": "Add Referral",
+          "referralAdded": "Referral added successfully",
           "searchPlaceholder": "Search by name, email, or Telegram ID...",
           "alreadyReferred": "already has referrer",
           "noUsersFound": "No users found"

src/locales/fa.json

@@ -2736,6 +2736,8 @@
           "noReferrals": "هنوز معرفی‌ای نیست",
           "removeReferral": "حذف معرفی",
           "referralRemoved": "معرفی حذف شد",
+          "addReferral": "افزودن معرفی",
+          "referralAdded": "معرفی اضافه شد",
           "searchPlaceholder": "جستجو بر اساس نام، ایمیل یا شناسه تلگرام...",
           "alreadyReferred": "قبلاً معرفی شده",
           "noUsersFound": "کاربری یافت نشد"

src/locales/ru.json

@@ -3629,6 +3629,8 @@
           "noReferrals": "Рефералов пока нет",
           "removeReferral": "Удалить реферала",
           "referralRemoved": "Реферал удалён",
+          "addReferral": "Добавить реферала",
+          "referralAdded": "Реферал добавлен",
           "searchPlaceholder": "Поиск по имени, email или Telegram ID...",
           "alreadyReferred": "уже чей-то реферал",
           "noUsersFound": "Пользователи не найдены"

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"