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/com/util/ViewHeader.tsx

@@ -49,6 +49,7 @@ export const ViewHeader = observer(function ViewHeader({
   return (
     <View style={[styles.header, pal.view]}>
       <TouchableOpacity
+        testID="viewHeaderBackOrMenuBtn"
         onPress={canGoBack ? onPressBack : onPressMenu}
         hitSlop={BACK_HITSLOP}
         style={canGoBack ? styles.backIcon : styles.backIconWide}>