admin: add connected mount clients page and dashboard section

The filer is the authority on who is subscribed to its metadata stream
(FUSE/VFS mounts, S3, peer filers, ...), but its in-memory listener
registry only tracked clientId->epoch and was not exposed.

- Enrich the filer subscriber registry with name/type/address/path/
  connected-time, populated in addClient and cleared in deleteClient so
  it reflects currently-connected clients only.
- Add a ListMetadataSubscribers filer gRPC (optional client-type filter).
- Admin server fans out to every filer, filters to mount types
  ("mount" Go weed mount, "sw-vfs" Rust VFS), and renders a new
  Cluster > Mount Clients page plus a Mount Clients dashboard section.

Read-only; no behavior change to the subscribe hot path.

weed/admin/dash/admin_data.go

@@ -38,6 +38,9 @@ type AdminData struct {
 	TotalEcVolumes int `json:"total_ec_volumes"` // Total number of EC volumes across all servers
 	TotalEcShards  int `json:"total_ec_shards"`  // Total number of EC shards across all servers
 
+	// TotalMountClients is the number of connected FUSE/VFS mount clients across filers.
+	TotalMountClients int `json:"total_mount_clients"`
+
 	// Trends holds at-a-glance sparklines built from the admin's own recent
 	// cluster snapshots (no Prometheus required).
 	Trends DashboardTrends `json:"trends"`
@@ -186,6 +189,12 @@ func (s *AdminServer) GetAdminData(username string) (AdminData, error) {
 	}
 	totalEcVolumes = len(ecVolumeSet)
 
+	// Count connected FUSE/VFS mount clients (best-effort; don't fail the dashboard)
+	totalMountClients := 0
+	if mountData, mountErr := s.GetMountClients(); mountErr == nil {
+		totalMountClients = mountData.TotalMountClients
+	}
+
 	// Prepare admin data
 	adminData := AdminData{
 		Username:          username,
@@ -202,6 +211,7 @@ func (s *AdminServer) GetAdminData(username string) (AdminData, error) {
 		LastUpdated:       topology.UpdatedAt,
 		TotalEcVolumes:    totalEcVolumes,
 		TotalEcShards:     totalEcShards,
+		TotalMountClients: totalMountClients,
 		Trends:            s.GetDashboardTrends(),
 	}