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/DTDesignSystemShowcase/Images.xcassets/AppIcon.appiconset/Contents.json

@@ -0,0 +1,14 @@
+{
+  "images": [
+    {
+      "filename": "App-Icon-1024x1024@1x.png",
+      "idiom": "universal",
+      "platform": "ios",
+      "size": "1024x1024"
+    }
+  ],
+  "info": {
+    "version": 1,
+    "author": "expo"
+  }
+}