admin/balance: fix Max Volumes display and balancer source selection (#8583)

* admin: fix Max Volumes column always showing 0

GetClusterVolumeServers() computed DiskCapacity from
diskInfo.MaxVolumeCount but never populated the MaxVolumes field
on the VolumeServer struct, causing the column to always display 0.

* balance: use utilization ratio for source server selection

The balancer selected the source server (to move volumes FROM) by raw
volume count. In clusters with heterogeneous MaxVolumeCount settings,
the server with the highest capacity naturally holds the most volumes
and was always picked as the source, even when it had the lowest
utilization ratio.

Change source selection and imbalance calculation to use utilization
ratio (effectiveCount / maxVolumeCount) so servers are compared by how
full they are relative to their capacity, not by absolute volume count.

This matches how destination scoring already works via
calculateBalanceScore().

weed/admin/dash/volume_management.go

@@ -457,6 +457,7 @@ func (s *AdminServer) GetClusterVolumeServers() (*ClusterVolumeServersData, erro
 
 						// Process disk information
 						for _, diskInfo := range node.DiskInfos {
+							vs.MaxVolumes += int(diskInfo.MaxVolumeCount)
 							vs.DiskCapacity += int64(diskInfo.MaxVolumeCount) * int64(volumeSizeLimitMB) * 1024 * 1024 // Use actual volume size limit
 
 							// Count regular volumes and calculate disk usage