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/en-US.json

@@ -937,8 +937,8 @@
       "deleteSuccess": "Cleared group from {count} client(s).",
       "resetTraffic": "Reset traffic",
       "resetConfirmTitle": "Reset traffic for group {name}?",
-      "resetConfirmContent": "This zeros up/down for all {count} client(s) in this group.",
-      "resetSuccess": "Reset traffic for {count} client(s).",
+      "resetConfirmContent": "This resets only the group's traffic counter. Individual client counters are not affected.",
+      "resetSuccess": "Group {name} traffic reset.",
       "adjustSuccess": "Adjusted {count} client(s) in {name}.",
       "emptyForAction": "This group has no clients yet.",
       "deleteGroupOnly": "Delete group (keep clients)",