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/mobile/ios/DTDesignSystemShowcase/Supporting/Expo.plist

@@ -0,0 +1,12 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
+<plist version="1.0">
+  <dict>
+    <key>EXUpdatesCheckOnLaunch</key>
+    <string>ALWAYS</string>
+    <key>EXUpdatesEnabled</key>
+    <false/>
+    <key>EXUpdatesLaunchWaitMs</key>
+    <integer>0</integer>
+  </dict>
+</plist>