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/lib/hooks/usePalette.ts

@@ -6,6 +6,7 @@ export interface UsePaletteValue {
   view: ViewStyle
   btn: ViewStyle
   border: ViewStyle
+  borderDark: ViewStyle
   text: TextStyle
   textLight: TextStyle
   textInverted: TextStyle
@@ -25,6 +26,9 @@ export function usePalette(color: PaletteColorName): UsePaletteValue {
     border: {
       borderColor: palette.border,
     },
+    borderDark: {
+      borderColor: palette.borderDark,
+    },
     text: {
       color: palette.text,
     },