feat: add demo mode flag

Related to https://kolaente.dev/vikunja/frontend/issues/2453

pkg/routes/api/v1/info.go

@@ -49,6 +49,7 @@ type vikunjaInfos struct {
 	EmailRemindersEnabled      bool      `json:"email_reminders_enabled"`
 	UserDeletionEnabled        bool      `json:"user_deletion_enabled"`
 	TaskCommentsEnabled        bool      `json:"task_comments_enabled"`
+	DemoModeEnabled            bool      `json:"demo_mode_enabled"`
 }
 
 type authInfo struct {
@@ -92,6 +93,7 @@ func Info(c echo.Context) error {
 		EmailRemindersEnabled:  config.ServiceEnableEmailReminders.GetBool(),
 		UserDeletionEnabled:    config.ServiceEnableUserDeletion.GetBool(),
 		TaskCommentsEnabled:    config.ServiceEnableTaskComments.GetBool(),
+		DemoModeEnabled:        config.ServiceDemoMode.GetBool(),
 		AvailableMigrators: []string{
 			(&vikunja_file.FileMigrator{}).Name(),
 			(&ticktick.Migrator{}).Name(),