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/src/components/ScreenContainer.tsx

@@ -0,0 +1,22 @@
+import React from 'react';
+import { ScrollView, StyleProp, ViewStyle } from 'react-native';
+import { useDTTheme } from '@dangerousthings/react-native';
+
+interface ScreenContainerProps {
+  children: React.ReactNode;
+  style?: StyleProp<ViewStyle>;
+}
+
+export function ScreenContainer({ children, style }: ScreenContainerProps) {
+  const theme = useDTTheme();
+
+  return (
+    <ScrollView
+      style={{ flex: 1, backgroundColor: theme.colors.background }}
+      contentContainerStyle={[{ padding: 24, paddingBottom: 48 }, style]}
+      showsVerticalScrollIndicator={false}
+    >
+      {children}
+    </ScrollView>
+  );
+}