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/vite.config.ts

@@ -0,0 +1,16 @@
+import { defineConfig } from 'vite';
+import path from 'path';
+
+export default defineConfig({
+  root: 'src/renderer',
+  base: './',
+  build: {
+    outDir: '../../dist/renderer',
+    emptyOutDir: true,
+  },
+  resolve: {
+    alias: {
+      '@dangerousthings/web': path.resolve(__dirname, '../../web/dist'),
+    },
+  },
+});