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/src/main/main.ts

@@ -0,0 +1,30 @@
+import { app, BrowserWindow } from 'electron';
+import path from 'path';
+
+function createWindow() {
+  const win = new BrowserWindow({
+    width: 1200,
+    height: 800,
+    backgroundColor: '#000000',
+    titleBarStyle: 'hiddenInset',
+    webPreferences: {
+      preload: path.join(__dirname, 'preload.js'),
+    },
+  });
+
+  if (process.env.VITE_DEV_SERVER_URL) {
+    win.loadURL(process.env.VITE_DEV_SERVER_URL);
+  } else {
+    win.loadFile(path.join(__dirname, '../renderer/index.html'));
+  }
+}
+
+app.whenReady().then(createWindow);
+
+app.on('window-all-closed', () => {
+  if (process.platform !== 'darwin') app.quit();
+});
+
+app.on('activate', () => {
+  if (BrowserWindow.getAllWindows().length === 0) createWindow();
+});