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/worker/tasks/balance/detection_test.go

@@ -852,3 +852,68 @@ func TestDetection_ExhaustedServerFallsThrough(t *testing.T) {
 	assertNoDuplicateVolumes(t, tasks)
 	t.Logf("Created %d tasks from node-b after node-a exhausted", len(tasks))
 }
+
+// TestDetection_HeterogeneousCapacity verifies that the balancer uses
+// utilization ratio (volumes/maxVolumes) rather than raw volume counts.
+// A server with more volumes but proportionally lower utilization should
+// NOT be picked as the source over a server with fewer volumes but higher
+// utilization.
+func TestDetection_HeterogeneousCapacity(t *testing.T) {
+	// Simulate a cluster like:
+	//   server-1: 600 volumes, max 700  → utilization 85.7%
+	//   server-2: 690 volumes, max 700  → utilization 98.6%  ← most utilized
+	//   server-3: 695 volumes, max 700  → utilization 99.3%  ← most utilized
+	//   server-4: 900 volumes, max 1260 → utilization 71.4%  ← least utilized
+	//
+	// The old algorithm (raw counts) would pick server-4 as source (900 > 695).
+	// The correct behavior is to pick server-3 (or server-2) as source since
+	// they have the highest utilization ratio.
+	servers := []serverSpec{
+		{id: "server-1", diskType: "hdd", dc: "dc1", rack: "rack1", maxVolumes: 700},
+		{id: "server-2", diskType: "hdd", dc: "dc1", rack: "rack1", maxVolumes: 700},
+		{id: "server-3", diskType: "hdd", dc: "dc1", rack: "rack1", maxVolumes: 700},
+		{id: "server-4", diskType: "hdd", dc: "dc1", rack: "rack1", maxVolumes: 1260},
+	}
+
+	volCounts := map[string]int{
+		"server-1": 600,
+		"server-2": 690,
+		"server-3": 695,
+		"server-4": 900,
+	}
+
+	var metrics []*types.VolumeHealthMetrics
+	vid := uint32(1)
+	for _, server := range []string{"server-1", "server-2", "server-3", "server-4"} {
+		count := volCounts[server]
+		metrics = append(metrics, makeVolumes(server, "hdd", "dc1", "rack1", "", vid, count)...)
+		vid += uint32(count)
+	}
+
+	at := buildTopology(servers, metrics)
+	clusterInfo := &types.ClusterInfo{ActiveTopology: at}
+	cfg := &Config{
+		BaseConfig:         base.BaseConfig{Enabled: true},
+		ImbalanceThreshold: 0.20,
+		MinServerCount:     2,
+	}
+
+	tasks, _, err := Detection(metrics, clusterInfo, cfg, 5)
+	if err != nil {
+		t.Fatalf("Detection failed: %v", err)
+	}
+	if len(tasks) == 0 {
+		t.Fatal("Expected balance tasks but got none")
+	}
+
+	// The source of the first task should be the most utilized server
+	// (server-3 at 99.3% or server-2 at 98.6%), NOT server-4.
+	firstSource := tasks[0].Server
+	if firstSource == "server-4" {
+		t.Errorf("Balancer incorrectly picked server-4 (lowest utilization 71.4%%) as source; should pick server-3 (99.3%%) or server-2 (98.6%%)")
+	}
+	if firstSource != "server-3" && firstSource != "server-2" {
+		t.Errorf("Expected server-3 or server-2 as first source, got %s", firstSource)
+	}
+	t.Logf("First balance task: move from %s (correct: highest utilization)", firstSource)
+}