fix(admin): 赠送流量配置项补全中英文案并修正语义描述

- 补提交 giftTraffic i18n key（此前仅表单落库，中文界面显示英文兜底）
- 修正描述：0=时长模式（按试用时长配置），>0=流量模式不限时用完为止
  （原文案「0=不限流量」与后端实际语义相反，会误导运营配置）

apps/admin/public/assets/locales/en-US/system.json

@@ -94,6 +94,8 @@
     "title": "Registration Settings",
     "trialConfig": "Trial Configuration",
     "trialConfigDescription": "Configure trial subscription, duration and time unit for new users upon registration",
+    "giftTraffic": "Gift Traffic",
+    "giftTrafficDescription": "Gift traffic for new users (GB). 0 = duration mode (uses the trial duration above); >0 = quota mode: N GB with no time limit, trial duration is ignored",
     "year": "Year(s)"
   },
   "site": {

apps/admin/public/assets/locales/zh-CN/system.json

@@ -94,6 +94,8 @@
     "title": "注册设置",
     "trialConfig": "试用配置",
     "trialConfigDescription": "配置新用户注册时的试用订阅、时长和时间单位",
+    "giftTraffic": "赠送流量",
+    "giftTrafficDescription": "新用户赠送流量（GB）。0 = 时长模式（按上方试用时长配置）；大于 0 = 流量模式：赠送 N GB、不限时长、用完为止（此时忽略试用时长配置）",
     "year": "年"
   },
   "site": {