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/.xcode.env

@@ -0,0 +1,11 @@
+# This `.xcode.env` file is versioned and is used to source the environment
+# used when running script phases inside Xcode.
+# To customize your local environment, you can create an `.xcode.env.local`
+# file that is not versioned.
+
+# NODE_BINARY variable contains the PATH to the node executable.
+#
+# Customize the NODE_BINARY variable here.
+# For example, to use nvm with brew, add the following line
+# . "$(brew --prefix nvm)/nvm.sh" --no-use
+export NODE_BINARY=$(command -v node)