Chromium-based desktop browsers allow to continue running background apps when browser is closed; this setting is found in System Settings. Is the Background Sync option located in the Site Settings section of the mobile version the same feature? If I disable Background Sync in Vanadium and close the tab, does it truly stop all activity from the web app? Additionally, what about JavaScript Service Workers - are they a privacy threat, and how do they function specifically within Vanadium?