feat: add gift subscription toggle to admin branding settings

Add CABINET_GIFT_ENABLED toggle in Admin Settings > Branding > Interface Options,
alongside existing fullscreen and email auth toggles. Includes updateGiftEnabled
API method and i18n keys for all 4 locales (ru/en/zh/fa).

src/locales/en.json

@@ -1696,6 +1696,8 @@
       "darkTheme": "Dark theme",
       "emailAuth": "Email auth",
       "emailAuthDesc": "Allow login via email",
+      "giftEnabled": "Gift subscription",
+      "giftEnabledDesc": "Allow users to gift a subscription to another user",
       "favoritesEmpty": "No favorites yet",
       "favoritesHint": "Star settings to add them here",
       "interfaceOptions": "Interface options",

src/locales/fa.json

@@ -1367,6 +1367,8 @@
       "darkTheme": "پوسته تیره",
       "emailAuth": "احراز هویت ایمیل",
       "emailAuthDesc": "اجازه ورود با ایمیل",
+      "giftEnabled": "اشتراک هدیه",
+      "giftEnabledDesc": "امکان ارسال اشتراک به عنوان هدیه به کاربر دیگر",
       "favoritesEmpty": "هنوز علاقه‌مندی ندارید",
       "favoritesHint": "ستاره بزنید تا تنظیمات اینجا اضافه شوند",
       "interfaceOptions": "گزینه‌های رابط کاربری",

src/locales/ru.json

@@ -1706,6 +1706,8 @@
       "autoFullscreenDesc": "В Telegram WebApp",
       "emailAuth": "Email авторизация",
       "emailAuthDesc": "Регистрация и вход через email",
+      "giftEnabled": "Подписка в подарок",
+      "giftEnabledDesc": "Возможность отправить подписку в подарок другому пользователю",
       "availableThemes": "Доступные темы",
       "darkTheme": "Тёмная",
       "lightTheme": "Светлая",

src/locales/zh.json

@@ -1405,6 +1405,8 @@
       "darkTheme": "暗色主题",
       "emailAuth": "邮箱认证",
       "emailAuthDesc": "允许通过邮箱登录",
+      "giftEnabled": "赠送订阅",
+      "giftEnabledDesc": "允许用户将订阅作为礼物赠送给其他用户",
       "favoritesEmpty": "暂无收藏",
       "favoritesHint": "点击星标将设置添加到这里",
       "interfaceOptions": "界面选项",