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/en-US.json

@@ -813,7 +813,8 @@
       "clientCount": "Clients in group",
       "totalGroups": "Total groups",
       "totalGroupedClients": "Clients with a group",
-      "emptyGroups": "Empty groups",
+      "trafficUsed": "Traffic used",
+      "totalTraffic": "Total traffic",
       "addGroup": "Add Group",
       "createSuccess": "Group \"{name}\" created.",
       "rename": "Rename",