fix: support --global on cron:set

`cron:set --global` wrote the property but emitted `unknown flag: --global` because the post-set `scheduler-cron-write` trigger received `--global` as the appName arg, which pflag rejected before reaching the trigger body.

The trigger args now omit appName for global writes, and the `scheduler-k3s` cron-write trigger short-circuits when called without an app since per-app reconciliation requires a real app name. The docker-local trigger already regenerates the global crontab from all apps so global `mailfrom`/`mailto` are picked up without any further changes.

plugins/scheduler-k3s/triggers.go

@@ -255,6 +255,10 @@ func TriggerSchedulerCronWrite(scheduler string, appName string) error {
 		return nil
 	}
 
+	if appName == "" {
+		return nil
+	}
+
 	cronTasks, err := cron.FetchCronTasks(cron.FetchCronTasksInput{AppName: appName})
 	if err != nil {
 		return fmt.Errorf("Error fetching cron tasks: %w", err)