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/api/adminBulkActions.ts

@@ -14,7 +14,8 @@ export type BulkActionType =
 
 export interface BulkActionRequest {
   action: BulkActionType;
-  user_ids: number[];
+  user_ids?: number[];
+  subscription_ids?: number[];
   params: BulkActionParams;
   dry_run?: boolean;
 }
@@ -48,6 +49,7 @@ export interface BulkProgressEvent {
   current: number;
   total: number;
   user_id: number;
+  subscription_id?: number;
   username?: string;
   success: boolean;
   message?: string;