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/controller/group.go

@@ -26,6 +26,7 @@ func (a *GroupController) initRouter(g *gin.RouterGroup) {
 	g.POST("/groups/create", a.create)
 	g.POST("/groups/rename", a.rename)
 	g.POST("/groups/delete", a.delete)
+	g.POST("/groups/resetTraffic", a.resetTraffic)
 	g.POST("/groups/bulkAdd", a.bulkAdd)
 	g.POST("/groups/bulkRemove", a.bulkRemove)
 }
@@ -108,6 +109,24 @@ func (a *GroupController) delete(c *gin.Context) {
 	notifyClientsChanged()
 }
 
+type groupResetTrafficBody struct {
+	Name string `json:"name"`
+}
+
+func (a *GroupController) resetTraffic(c *gin.Context) {
+	var body groupResetTrafficBody
+	if err := c.ShouldBindJSON(&body); err != nil {
+		jsonMsg(c, I18nWeb(c, "somethingWentWrong"), err)
+		return
+	}
+	if err := a.clientService.ResetGroupTraffic(body.Name); err != nil {
+		jsonMsg(c, I18nWeb(c, "somethingWentWrong"), err)
+		return
+	}
+	jsonObj(c, gin.H{"name": body.Name}, nil)
+	notifyClientsChanged()
+}
+
 type bulkAddToGroupRequest struct {
 	Emails []string `json:"emails"`
 	Group  string   `json:"group"`