Add desktop (Electron) and mobile (Expo) showcase apps

New workspace packages for demonstrating design system components:
- Desktop: Electron + Vite app with brand/theme switching
- Mobile: Expo React Native app with brand switching

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/showcase/mobile/index.js

@@ -0,0 +1,4 @@
+import { registerRootComponent } from "expo";
+import App from "./App";
+
+registerRootComponent(App);