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

@@ -201,6 +201,7 @@ func (d DockerHost) ToFrontend() DockerHostFrontend {
 		AgentID:           d.AgentID,
 		Hostname:          d.Hostname,
 		DisplayName:       d.DisplayName,
+		CustomDisplayName: d.CustomDisplayName,
 		MachineID:         d.MachineID,
 		OS:                d.OS,
 		KernelVersion:     d.KernelVersion,

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"`