feat(groups): reset group traffic without touching client counters

The group page shows traffic counting per group, but the only reset
available zeroed every member client's up/down counters (and their
quotas) via bulkResetTraffic. Group traffic is a derived sum of client
traffic, so zeroing the group display previously required mutating the
clients themselves.

Add a display-only baseline: ClientGroup gains reset_up/reset_down
columns (additive, handled by AutoMigrate). ResetGroupTraffic snapshots
the group's current up/down sum into the baseline, and ListGroups now
reports max(0, sum - baseline). Client counters are left untouched and
no Xray restart is triggered. A new POST /panel/api/clients/groups/
resetTraffic endpoint drives it, creating the client_groups row when the
group exists only as a derived label.

The groups page action now calls the new endpoint; confirm/success
strings updated across all 13 locales to reflect group-only semantics.

internal/web/translation/pt-BR.json

@@ -934,8 +934,8 @@
       "deleteSuccess": "Grupo limpo de {count} cliente(s).",
       "resetTraffic": "Redefinir tráfego",
       "resetConfirmTitle": "Redefinir tráfego do grupo {name}?",
-      "resetConfirmContent": "Isso zera up/down para todos os {count} cliente(s) deste grupo.",
-      "resetSuccess": "Tráfego redefinido para {count} cliente(s).",
+      "resetConfirmContent": "Isso redefine apenas o contador de tráfego do grupo. Os contadores de cada cliente não são afetados.",
+      "resetSuccess": "Tráfego do grupo {name} redefinido.",
       "adjustSuccess": "Ajustados {count} cliente(s) em {name}.",
       "emptyForAction": "Este grupo ainda não tem clientes.",
       "deleteGroupOnly": "Excluir grupo (manter clientes)",