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/ios/Podfile.properties.json

@@ -0,0 +1,5 @@
+{
+  "expo.jsEngine": "hermes",
+  "EX_DEV_CLIENT_NETWORK_INSPECTOR": "true",
+  "newArchEnabled": "true"
+}