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/android/build.gradle

@@ -0,0 +1,24 @@
+// Top-level build file where you can add configuration options common to all sub-projects/modules.
+
+buildscript {
+  repositories {
+    google()
+    mavenCentral()
+  }
+  dependencies {
+    classpath('com.android.tools.build:gradle')
+    classpath('com.facebook.react:react-native-gradle-plugin')
+    classpath('org.jetbrains.kotlin:kotlin-gradle-plugin')
+  }
+}
+
+allprojects {
+  repositories {
+    google()
+    mavenCentral()
+    maven { url 'https://www.jitpack.io' }
+  }
+}
+
+apply plugin: "expo-root-project"
+apply plugin: "com.facebook.react.rootproject"