feat(groups): show used traffic per group in groups table

Sum up+down across each group's clients via a LEFT JOIN on email in
ListGroups, expose it as trafficUsed on GroupSummary, and render it as a
new column plus a "Total traffic" summary card. Drops the unused "Empty
groups" card and its translation key.

web/translation/fa-IR.json

@@ -812,7 +812,8 @@
       "clientCount": "کاربران در گروه",
       "totalGroups": "تعداد گروه‌ها",
       "totalGroupedClients": "کاربران دارای گروه",
-      "emptyGroups": "گروه‌های خالی",
+      "trafficUsed": "ترافیک مصرف‌شده",
+      "totalTraffic": "مجموع ترافیک",
       "addGroup": "افزودن گروه",
       "createSuccess": "گروه «{name}» ایجاد شد.",
       "rename": "تغییر نام",