feat: add promo group and promo offer management to AdminUserDetail

- Info tab: promo group card with inline dropdown to change/remove group
- Balance tab: active promo offer display with deactivate button
- Balance tab: send promo offer form (discount %, valid hours)
- Add deactivateDiscount API method to promocodesApi
- Add locale keys for ru/en/zh/fa
- Fix race condition in subscription create (stale state read)
- Fix unhandled promise in handleInlineConfirm
- Remove unused legacy adminUsers block from ru.json

src/locales/en.json

@@ -1886,6 +1886,22 @@
           "title": "Actions",
           "areYouSure": "Are you sure?"
         },
+        "promoGroup": "Promo group",
+        "noPromoGroup": "Not set",
+        "changePromoGroup": "Change",
+        "selectPromoGroup": "Select group",
+        "removePromoGroup": "Remove",
+        "activePromoOffer": "Active promo offer",
+        "discount": "Discount",
+        "source": "Source",
+        "expiresAt": "Expires",
+        "deactivateOffer": "Deactivate",
+        "sendOffer": "Send offer",
+        "discountPercent": "Discount %",
+        "validHours": "Valid hours",
+        "offerSent": "Offer sent",
+        "offerSendError": "Failed to send offer",
+        "offerDeactivated": "Offer deactivated",
         "subscription": {
           "current": "Current subscription",
           "tariff": "Tariff",