Summary: Settings tab should be a modal sheet to improve navigation. This is motivated by two observations: 1) Opening Settings is a temporary diversion in the user's workflow, after which the user will want to return where she left off. Since Settings are a separate tab, the user must think about which tab to return to, find it in the tab bar, etc. Having a modal sheet that can simply be dismissed, returning the user to their workflow, is a more natural UI. 2) Firefox and Chrome both present Settings as a modal sheet.