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/es-ES.json

@@ -934,8 +934,8 @@
       "deleteSuccess": "Grupo limpiado de {count} cliente(s).",
       "resetTraffic": "Restablecer tráfico",
       "resetConfirmTitle": "¿Restablecer tráfico del grupo {name}?",
-      "resetConfirmContent": "Esto pone a cero up/down para los {count} cliente(s) de este grupo.",
-      "resetSuccess": "Tráfico restablecido en {count} cliente(s).",
+      "resetConfirmContent": "Esto restablece solo el contador de tráfico del grupo. Los contadores de cada cliente no se ven afectados.",
+      "resetSuccess": "Tráfico del grupo {name} restablecido.",
       "adjustSuccess": "Ajustados {count} cliente(s) en {name}.",
       "emptyForAction": "Este grupo aún no tiene clientes.",
       "deleteGroupOnly": "Eliminar grupo (conservar clientes)",