feat(groups): show upload/download breakdown in group traffic

Add per-group up/down to GroupSummary (backend + schema), surface them
as Upload/Download columns in the groups table, and fold upload/download
into the Total traffic summary card. Rename the group "Clients in group"
column to just "Clients" across all locales.

internal/web/translation/zh-CN.json

@@ -812,10 +812,12 @@
     "groups": {
       "title": "分组",
       "name": "名称",
-      "clientCount": "分组中的客户端",
+      "clientCount": "客户端",
       "totalGroups": "分组总数",
       "totalGroupedClients": "有分组的客户端",
       "trafficUsed": "已用流量",
+      "upload": "上传",
+      "download": "下载",
       "totalTraffic": "总流量",
       "addGroup": "添加分组",
       "createSuccess": "已创建分组 “{name}”。",