refactor: align tariff builder with bot functionality

- Remove unused fields: custom_days, custom_traffic (bot doesn't use them)
- Add promo groups selection to tariff form
- Add API endpoint for fetching available promo groups
- Make traffic topup packages dynamic (add/remove any number)
- Fix stuck 0 in number inputs when clearing
- Add validation messages explaining why save button is blocked
- Match bot validation: name 2-50 chars, device_limit >= 1, tier_level 1-10
- Add missing i18n translations for new fields and validation errors

src/api/tariffs.ts

@@ -236,4 +236,10 @@ export const tariffsApi = {
     const response = await apiClient.get('/cabinet/admin/tariffs/available-servers');
     return response.data;
   },
+
+  // Get available promo groups for selection
+  getAvailablePromoGroups: async (): Promise<{ id: number; name: string }[]> => {
+    const response = await apiClient.get('/cabinet/admin/payment-methods/promo-groups');
+    return response.data;
+  },
 };