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/database/model/model.go

@@ -639,6 +639,8 @@ func (ClientRecord) TableName() string { return "clients" }
 type ClientGroup struct {
 	Id        int    `json:"id" gorm:"primaryKey;autoIncrement"`
 	Name      string `json:"name" gorm:"uniqueIndex;not null"`
+	ResetUp   int64  `json:"resetUp" gorm:"column:reset_up;default:0"`
+	ResetDown int64  `json:"resetDown" gorm:"column:reset_down;default:0"`
 	CreatedAt int64  `json:"createdAt" gorm:"autoCreateTime:milli"`
 	UpdatedAt int64  `json:"updatedAt" gorm:"autoUpdateTime:milli"`
 }