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/desktop/electron-builder.yml

@@ -0,0 +1,18 @@
+appId: com.dangerousthings.showcase-desktop
+productName: DT Design System Showcase
+directories:
+  output: release
+  buildResources: resources
+npmRebuild: false
+artifactName: "dt-showcase-desktop-${version}-${arch}.${ext}"
+files:
+  - dist/**/*
+  - package.json
+  - "!node_modules"
+mac:
+  target: [dmg, zip]
+win:
+  target: [nsis, portable]
+linux:
+  target: [AppImage, deb]
+  category: Development