fix(clients): keep a client editable when its subId is already shared (#6065)

The subId collision check in Update ran on every save, unlike the email
check above it. Because Update defaults an omitted subId to the stored
one, any client already sharing a subId was rejected on every later edit
-- even a pure totalGB or expiry change that never mentions subId.

Gate the check on an actual change. Pre-existing duplicates are reachable
because SyncInbound has no such check, and 88a36773 meant to leave them
untouched. Editing a client onto another subscriber's subId is still
rejected, so the typo guard is intact.

internal/web/service/client_crud.go

@@ -375,7 +375,7 @@ func (s *ClientService) Update(inboundSvc *InboundService, id int, updated model
 		}
 	}
 
-	if updated.SubID != "" {
+	if updated.SubID != existing.SubID {
 		var subCollision int64
 		if err := database.GetDB().Model(&model.ClientRecord{}).
 			Where("sub_id = ? AND id <> ?", updated.SubID, id).