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/App.tsx

@@ -0,0 +1,45 @@
+import React from 'react';
+import { ActivityIndicator, View } from 'react-native';
+import { useFonts } from 'expo-font';
+import { NavigationContainer } from '@react-navigation/native';
+import { DTThemeProvider } from '@dangerousthings/react-native';
+import { BrandProvider, useBrand } from './src/brand/BrandContext';
+import { RootNavigator } from './src/navigation/RootNavigator';
+import { useNavigationTheme } from './src/navigation/navigationTheme';
+
+function AppContent() {
+  const { theme } = useBrand();
+  const navigationTheme = useNavigationTheme();
+
+  return (
+    <DTThemeProvider theme={theme}>
+      <NavigationContainer theme={navigationTheme}>
+        <RootNavigator />
+      </NavigationContainer>
+    </DTThemeProvider>
+  );
+}
+
+export default function App() {
+  const [fontsLoaded] = useFonts({
+    Tektur: require('./assets/fonts/Tektur-VariableFont_wdth,wght.ttf'),
+    'Tektur-Regular': require('./assets/fonts/Tektur-Regular.ttf'),
+    'Tektur-Medium': require('./assets/fonts/Tektur-Medium.ttf'),
+    'Tektur-SemiBold': require('./assets/fonts/Tektur-SemiBold.ttf'),
+    'Tektur-Bold': require('./assets/fonts/Tektur-Bold.ttf'),
+  });
+
+  if (!fontsLoaded) {
+    return (
+      <View style={{ flex: 1, backgroundColor: '#000', justifyContent: 'center', alignItems: 'center' }}>
+        <ActivityIndicator size="large" color="#00FFFF" />
+      </View>
+    );
+  }
+
+  return (
+    <BrandProvider initialBrand="dt">
+      <AppContent />
+    </BrandProvider>
+  );
+}