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/vi-VN.json

@@ -812,10 +812,12 @@
     "groups": {
       "title": "Nhóm",
       "name": "Tên",
-      "clientCount": "Client trong nhóm",
+      "clientCount": "Client",
       "totalGroups": "Tổng số nhóm",
       "totalGroupedClients": "Client có nhóm",
       "trafficUsed": "Lưu lượng đã dùng",
+      "upload": "Tải lên",
+      "download": "Tải xuống",
       "totalTraffic": "Tổng lưu lượng",
       "addGroup": "Thêm nhóm",
       "createSuccess": "Đã tạo nhóm «{name}».",