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/en.json

@@ -2095,7 +2095,17 @@
         "daysRemaining": "Days",
         "promoGroup": "Group",
         "spent": "Spent"
-      }
+      },
+      "subscriptionsSelected": "{{count}} subscriptions selected",
+      "usersSelected": "{{count}} users selected",
+      "expandSubscriptions": "Show subscriptions",
+      "collapseSubscriptions": "Hide subscriptions",
+      "noSubscriptions": "No subscriptions",
+      "subscriptionTarget": "Target: subscriptions",
+      "userTarget": "Target: users",
+      "daysUnit": "d",
+      "trafficOf": "of",
+      "trafficGbUnit": "GB"
     },
     "theme": {
       "accentColor": "Accent color",