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/id-ID.json

@@ -812,7 +812,8 @@
       "clientCount": "Klien di grup",
       "totalGroups": "Total grup",
       "totalGroupedClients": "Klien dengan grup",
-      "emptyGroups": "Grup kosong",
+      "trafficUsed": "Trafik terpakai",
+      "totalTraffic": "Total trafik",
       "addGroup": "Tambah grup",
       "createSuccess": "Grup «{name}» dibuat.",
       "rename": "Ubah nama",