feat: Add empty response handling to services and subscribers list commands with sample data fallback

- Update listServices to check for empty services slice in addition to error condition
- Update listSubscribers to check for empty subscribers slice in addition to error condition
- Add muted message when API returns empty results but no error
- Preserve existing warning message when API returns error
- Show sample/placeholder data in both empty and error cases

apps/cli/internal/commands/services.go

@@ -59,8 +59,12 @@ func listServices(u *uiContext) error {
 	t.AddColumn("Version", 10, "left")
 	t.AddColumn("Uptime", 10, "right")
 
-	if err != nil {
-		u.warn("Using placeholder data: " + err.Error())
+	if err != nil || len(services) == 0 {
+		if err != nil {
+			u.warn("Using placeholder data: " + err.Error())
+		} else {
+			u.muted("(API returned no services - showing sample data)")
+		}
 		placeholders := [][]string{
 			{"api-server", "Running", "v1.0.0", "2h15m"},
 			{"charging-engine", "Running", "v1.0.0", "2h15m"},

apps/cli/internal/commands/subscribers.go

@@ -52,8 +52,12 @@ func listSubscribers(u *uiContext) error {
 	u.header("Active Subscribers")
 
 	subs, err := u.client.ListSubscribers()
-	if err != nil {
-		u.warn("Using placeholder data: " + err.Error())
+	if err != nil || len(subs) == 0 {
+		if err != nil {
+			u.warn("Using placeholder data: " + err.Error())
+		} else {
+			u.muted("(API returned no subscribers - showing sample data)")
+		}
 		t := u.newTable()
 		t.AddColumn("IMSI", 16, "left")
 		t.AddColumn("Name", 22, "left")