Fix Docker host custom display name not persisting in UI (related to #662)

The custom display name feature added in cd627f33c had a critical bug where
the backend successfully stored custom names but the frontend never received
them, making the feature appear non-functional.

Root cause:
- DockerHost.CustomDisplayName was stored in backend state (models.go:201)
- SetDockerHostCustomDisplayName() correctly updated the field
- BUT DockerHostFrontend struct was missing customDisplayName field
- AND ToFrontend() converter didn't copy CustomDisplayName
- Result: WebSocket state broadcasts stripped out the custom name

When users edited a Docker host display name:
- API returned 200 OK ✓
- Success notification appeared ✓
- Edit state cleared ✓
- But subsequent state broadcasts lacked customDisplayName ✗
- UI continued showing original name ✗

Fix:
- Add CustomDisplayName field to DockerHostFrontend (models_frontend.go:105)
- Copy d.CustomDisplayName in ToFrontend() converter (converters.go:204)
- Now custom display names properly propagate to frontend via WebSocket

The feature now works as originally intended - custom names persist across
agent reconnections and display correctly in the UI.

internal/models/converters.go

@@ -197,26 +197,27 @@ func (c Container) ToFrontend() ContainerFrontend {
 // ToFrontend converts a DockerHost to DockerHostFrontend
 func (d DockerHost) ToFrontend() DockerHostFrontend {
 	h := DockerHostFrontend{
-		ID:               d.ID,
-		AgentID:          d.AgentID,
-		Hostname:         d.Hostname,
-		DisplayName:      d.DisplayName,
-		MachineID:        d.MachineID,
-		OS:               d.OS,
-		KernelVersion:    d.KernelVersion,
-		Architecture:     d.Architecture,
-		Runtime:          d.Runtime,
-		RuntimeVersion:   d.RuntimeVersion,
-		DockerVersion:    d.DockerVersion,
-		CPUs:             d.CPUs,
-		TotalMemoryBytes: d.TotalMemoryBytes,
-		UptimeSeconds:    d.UptimeSeconds,
-		CPUUsagePercent:  d.CPUUsage,
-		Status:           d.Status,
-		LastSeen:         d.LastSeen.Unix() * 1000,
-		IntervalSeconds:  d.IntervalSeconds,
-		AgentVersion:     d.AgentVersion,
-		Containers:       make([]DockerContainerFrontend, len(d.Containers)),
+		ID:                d.ID,
+		AgentID:           d.AgentID,
+		Hostname:          d.Hostname,
+		DisplayName:       d.DisplayName,
+		CustomDisplayName: d.CustomDisplayName,
+		MachineID:         d.MachineID,
+		OS:                d.OS,
+		KernelVersion:     d.KernelVersion,
+		Architecture:      d.Architecture,
+		Runtime:           d.Runtime,
+		RuntimeVersion:    d.RuntimeVersion,
+		DockerVersion:     d.DockerVersion,
+		CPUs:              d.CPUs,
+		TotalMemoryBytes:  d.TotalMemoryBytes,
+		UptimeSeconds:     d.UptimeSeconds,
+		CPUUsagePercent:   d.CPUUsage,
+		Status:            d.Status,
+		LastSeen:          d.LastSeen.Unix() * 1000,
+		IntervalSeconds:   d.IntervalSeconds,
+		AgentVersion:      d.AgentVersion,
+		Containers:        make([]DockerContainerFrontend, len(d.Containers)),
 	}
 
 	if h.DisplayName == "" {

internal/models/models_frontend.go

@@ -102,6 +102,7 @@ type DockerHostFrontend struct {
 	AgentID           string                     `json:"agentId"`
 	Hostname          string                     `json:"hostname"`
 	DisplayName       string                     `json:"displayName"`
+	CustomDisplayName string                     `json:"customDisplayName,omitempty"`
 	MachineID         string                     `json:"machineId,omitempty"`
 	OS                string                     `json:"os,omitempty"`
 	KernelVersion     string                     `json:"kernelVersion,omitempty"`