Account switcher (#85)

* Update the account-create and signin views to use the design system.

Also:
- Add borderDark to the theme
- Start to an account selector in the signin flow

* Dark mode fixes in signin ui

* Track multiple active accounts and provide account-switching UI

* Add test tooling for an in-memory pds

* Add complete integration tests for login and the account switcher

src/view/shell/mobile/Menu.tsx

@@ -62,7 +62,7 @@ export const Menu = observer(
       onPress?: () => void
     }) => (
       <TouchableOpacity
-        testID="menuItemButton"
+        testID={`menuItemButton-${label}`}
         style={styles.menuItem}
         onPress={onPress ? onPress : () => onNavigate(url || '/')}>
         <View style={[styles.menuItemIconWrapper]}>