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/ThemeContext.tsx

@@ -18,6 +18,7 @@ export type PaletteColor = {
   textInverted: string
   link: string
   border: string
+  borderDark: string
   icon: string
   [k: string]: string
 }