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>

turbo.json

@@ -5,6 +5,16 @@
       "dependsOn": ["^build"],
       "outputs": ["dist/**"]
     },
+    "dev": {
+      "dependsOn": ["^build"],
+      "cache": false,
+      "persistent": true
+    },
+    "start": {
+      "dependsOn": ["^build"],
+      "cache": false,
+      "persistent": true
+    },
     "clean": {
       "cache": false
     },