feat(composer): carry reference image names and submit on Enter

Propagate an optional imageName through the agent content pipeline
(frontend types → API → domain → prompt building) so reference images
render with their name instead of a bare "image" label.

Also switch PromptComposer to submit on plain Enter (Shift+Enter for
newline, IME-safe) and add a submitDisabled guard wired from the canvas
and home composers.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

server/internal/application/design_service.go

@@ -4510,7 +4510,13 @@ func promptFromAgentMessages(messages []design.AgentChatMessage) string {
 					if builder.Len() > 0 {
 						builder.WriteString("\n")
 					}
-					builder.WriteString("Reference image: ")
+					builder.WriteString("Reference image")
+					if imageName := strings.TrimSpace(content.ImageName); imageName != "" {
+						builder.WriteString(" (")
+						builder.WriteString(compactReferenceImageName(imageName))
+						builder.WriteString(")")
+					}
+					builder.WriteString(": ")
 					builder.WriteString(imageURL)
 				}
 			}
@@ -4519,6 +4525,10 @@ func promptFromAgentMessages(messages []design.AgentChatMessage) string {
 	return builder.String()
 }
 
+func compactReferenceImageName(value string) string {
+	return strings.Join(strings.Fields(value), " ")
+}
+
 func hasUserPromptTextInAgentMessages(messages []design.AgentChatMessage) bool {
 	for _, message := range messages {
 		if strings.TrimSpace(message.Role) != "" && strings.ToLower(strings.TrimSpace(message.Role)) != "user" {