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/id-ID.json

@@ -934,8 +934,8 @@
       "deleteSuccess": "Grup dihapus dari {count} klien.",
       "resetTraffic": "Reset trafik",
       "resetConfirmTitle": "Reset trafik grup {name}?",
-      "resetConfirmContent": "Ini mengatur ulang up/down ke 0 untuk semua {count} klien di grup ini.",
-      "resetSuccess": "Trafik direset untuk {count} klien.",
+      "resetConfirmContent": "Ini hanya mengatur ulang penghitung trafik grup. Penghitung tiap klien tidak terpengaruh.",
+      "resetSuccess": "Trafik grup {name} direset.",
       "adjustSuccess": "{count} klien di {name} disesuaikan.",
       "emptyForAction": "Grup ini belum memiliki klien.",
       "deleteGroupOnly": "Hapus grup (pertahankan klien)",