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/tsconfig.main.json

@@ -0,0 +1,14 @@
+{
+  "extends": "../../../tsconfig.base.json",
+  "compilerOptions": {
+    "outDir": "dist/main",
+    "rootDir": "src/main",
+    "module": "CommonJS",
+    "moduleResolution": "node",
+    "lib": ["ES2022"],
+    "declaration": false,
+    "declarationMap": false,
+    "sourceMap": false
+  },
+  "include": ["src/main"]
+}