feat: multi-tariff bulk actions UI — subscription-level selection

- Detect multi-tariff mode when users have multiple subscriptions
- Expandable user rows with chevron: click to show subscription sub-rows
- Each subscription sub-row shows: tariff name, status badge, days
  remaining (color-coded green/amber/red), traffic progress bar
- Independent subscription checkboxes for subscription-level actions
- FloatingActionBar shows dual counters: users (accent) + subscriptions
  (green), actions grouped by target type with disabled state
- Subscription-level actions send subscription_ids, user-level send
  user_ids to the backend
- Backward compatible: single-tariff mode unchanged (no chevrons,
  no sub-rows, user-only selection)
- i18n: subscriptionsSelected, usersSelected, expand/collapse,
  daysUnit, trafficOf, trafficGbUnit (ru + en)

src/locales/ru.json

@@ -3843,7 +3843,17 @@
         "daysRemaining": "Дни",
         "promoGroup": "Группа",
         "spent": "Потрачено"
-      }
+      },
+      "subscriptionsSelected": "{{count}} подписок выбрано",
+      "usersSelected": "{{count}} пользователей выбрано",
+      "expandSubscriptions": "Показать подписки",
+      "collapseSubscriptions": "Скрыть подписки",
+      "noSubscriptions": "Нет подписок",
+      "subscriptionTarget": "Цель: подписки",
+      "userTarget": "Цель: пользователи",
+      "daysUnit": "дн.",
+      "trafficOf": "из",
+      "trafficGbUnit": "ГБ"
     },
     "theme": {
       "accentColor": "Акцентный цвет",